Let's see if I can summarize my original intent and what has been learned...

PRIMARY GOAL: Use the 404SX as a virtual backing band with no limitation on the total length of the song. Arrangement and length are predetermined.
SECONDARY GOAL: Break from fixed arrangement and length only to the extent of having a flexible end of the song to allow for jams. Bulk of song is still fixed.

RULES & CONSTRAINTS: No sequencer. MIDI start/stop/beat clock allowed. Tempo of Adrenalinn III (AD3) must match the virtual backing band.

PROBLEMS: 404 has a 99 bar limit for patterns (related to primary goal). A break or glitch in audio can be heard when switching from one pattern to another (related to secondary goal).

MY SPECIFIC SETUP: Two players. Electric guitar through AD3. SP404SX. AD3 MIDI out to 404 MIDI in. Electric bass. Two vocals

I think dman5000 has the solution to the first issue / primary goal. If the entire song (of fixed arrangement and length) is loaded into a 404 pad (multiple pads OK if there are multitrack files), that pad can be loaded into a 99-bar pattern and started via MIDI. The 404's tempo should be set to 40 (allowing for a song well over 9 minutes). MIDI beat clock should not be sent. THIS IS KEY. I don't believe the 404 can ignore beat clock AND still receive start/stop. If the beat clock is received the 404 will set its tempo accordingly and the length of the song will be shortened accordingly.

Jim Y and PHeMoX have some suggestions on the second problem / secondary goal. My approach was to split the song into a main section and a jam (looping) section. Put each into a pattern. Queue up the first pattern (HOLD+) and trigger with the AD3 start message. Once it starts (a few beats of silence at the beginning) hit the next pattern. This queues up the second pattern to play when the first one is done (the second pattern would loop indefinitely -- the nature of the 404). Play away... Functionally, this works. However, a slight interruption in audio is heard when the patterns switch. It's been suggested that either the 404 simply cannot smoothly switch between patterns or that there's some drift going on in the playback of the first section that results in an undesirable gap or overlap when the patterns switch. Guaranteeing a cross fade or zero-crossing at the end has been suggested. Keeping the sections in pads and triggering via foot switch has been suggested.

By the way, I do have an FCB1010 but am not using it. I didn't think it put out start/stop.

I haven't given up and really appreciate all the comments. I'll report more when I know more.

Thanks, that's pretty clear.
I don't think the 1010 board can send start/stop either, but if the AD3 has a start/stop footswitch jack, the 1010 might be programmed to operate it via it's relay switch outputs. (If it doesn't have a start/stop jack, it may be easy to mod it by wiring a jack to the built in switch).

Thinking differently, if you can live with fixed song length, a digital multitrack recorder that has start/stop footswitch jack and can send midi clock, could control the AD3 and with that, it's tempo would be in sync to the audio track. That leaves the 404 with little to do of course though it could provide ambients and filler loops for between songs.
You could have different length versions of the songs on the recorder to give you at least some performance options.
I have an old Roland VS840 recorder could work this and could even have tempo changes during a song because it supports midi tempo maps. Downside is it's a bit slow loading songs - which might not be a problem if there's someone up there with you to keep the audience entertained while you cue the next song up. The VS840 is old now (I wouldn't consider it due to an unreliable Zip drive unless it has been modded to use flash memory like mine has), but I'm sure something more modern is available.

Jim

OK, the Adrenalinn thing didn't work. If you tell it to turn off its beat clock it also turns off start/stop commands. So, no way to trigger a 404 pattern and have the 404 use its own tempo (40 bpm in the case I was discussing to maximize song length). I'm stuck with the 99 bar pattern limit. At least with my current setup. If there were another piece of gear that only put out start/stop...?

Now, on to cascading patterns. I was able to avoid the glitch by choosing a more favorable edit point. But then two different things happened. The first time it transitioned from pattern 1 to pattern 2 everything went smoothly. But the bars kept counting (no reset back to 1). When I hit 99, even though the 404 was playing the second pattern, it went back to the beginning of pattern 2! Weird. When I tried the whole experiment again there was suddenly a huge gap (quarter second?) transitioning between patterns. Grrr. But the bar counter did reset.

I think I need to add something to my setup to actually make this work -- a sequencer. Now, I've been withholding information from you fine folks here. I own a Kurzweil PC361, which possesses a more-than-capable sequencer. I just don't want to lug the thing around just for that! I'm kind of looking around for a really cheap sequencer. All I really need it to do is the the 404 to play a sample and send out a beat clock to the Adrenalinn.

One last observation... When I loaded 9 multitrack samples (6 mono, 3 stereo) in and triggered them all simultaneously in a pattern it sounded as if the timing of the tracks were varying. Real jerky sounding. Like the 404 can't quite fire all 12 of its voices simultaneously. If I load a full stereo mix into a pad the problem goes away.

I'm not hatin' on the 404. I did my research. I don't think I could have known then what I know now without buying it. If I can find a cheap sequencer or lug my Kurz around the 404 will continue to be part of my backing track solution.

Cheapest sequencer would be a drum machine. Most can have patterns programmed into a song structure and there's no reason patterns cannot be empty if you have to exceed the max length of a single pattern, so you should be able to use the correct BPM. Yamaha QY sequencers should also be suitable.

I still think you'll have trouble with the 404 playback duration not matching the bar count duration and the BPM of the sample getting progressively out of time with the midi clock - there is no sync between the audio and midi, even if they start at the same time
Jim.

Just thought I'd chime in with my final solution...

I load a full stereo mix of my entire song into a single pad of the 404. My keyboard (Kurzweil PC361) has a sequence loaded in it with a single quarter note at the top to trigger that pad.

The keyboard actually has two MIDI outs -- one goes to the 404 and one goes to the Adrenalinn (the Adrenalinn has an OUT but it doesn't do THRU). Hit start on the keyboard and the 404 starts along with the MIDI beat clock for the Adrenalinn.

Yes, I know they are operating independently (audio track and MIDI clock) but it seems to hold together over the course of the song. I'm calling that "good enough" for the gear we currently have and what we're trying to accomplish.

A million thanks to all those that chimed in with ideas. I learned a lot in this journey.