I've generally stuck to one of two strategies at any given time:
- We agree I set the tempo. They can be any tempo at any time. I have to turn a knob or plan a change in advance. Therefore, they have the flexibility and I do not. So, when I start, they go from there. In a sense, the song becomes the click track.
- Free improv: No one bothers with tempo too much and we just make cool sounds.
If you time it right, you can have both. One thing I like about the Polyend Tracker as a sequencer using MIDI to CV conversion (or a Nerdseq will do it) is you can program this behavior to happen at specific times and even program sequences that are non-timed.
I think the issue with a tempo-tap based system is you have to always have some rhythmic element that sends steady clock to stuff. You can maybe plan out sections with this, or even an envelope follower depending on what you're doing.
The only option for you I can think of is to think less in terms of melody and more in terms of percussion or just "triggering events". If you set the modular up to respond to drum triggers from you, or maybe some sort of Launchpad/other pad based finger drumming setup, then you'll be fine and you just have to worry about tuning and setting up the patching in advance (relatively speaking) so you can have at least one hand free to trigger stuff.
You could also try the Korg SQ-64, which has a play mode called "isomorphic" where the 64 pads roughly emulate the layout of a bass. The Korg SQ-64 has had a mixed reputation, but it is powerful on paper and may work for you here so you can apply your guitar/bass skills in this context.