Appreciate it could be tricky, thanks for the reply!
The way I imagined it working would be I'd create an audio click track using a very distinct defined sound (like a sine wave beep), accents (ie. first beat of the bar) would be at full strength, the rest of the beats at something less, say 60%. The app would simply have a 'beat indicator' icon..if there is no signal, it's black, if there is full signal, it's white (for the duration of the signal), if there's less than full signal, it's either some shade of grey, or bases it's shade of gray from the strength of the signal. The click track is already in sync with the rest of the tracks (that's up to me to make sure when rendering out my tracks). All it would be doing is outputting a colour based on the strength of the signal as it plays through. Maybe there could be some kind of offset that the user can dial in in case it ends up being slightly behind or ahead.
Another idea could be to have a midi file as one of the multitrack tracks that has a note for each beat, and have that output midi alongside the audio tracks. That could be fed into a midi device which could be programmed to flash a light (my synth actually has a little midi status LED on it that flashes when it receives any midi input).