Those two approaches make sense. I imagine the free improve approach could be somewhat dependent on the types of sounds/effects being coaxed out of the modular. If the primary rhythmic component of the synth sound is just something like some mellow filter modulation, for example, it might be harder for the others to lock into it, compared to a more heavily sequenced groove. Or, if the synth part is more like an intro that fades as the band comes in, tempo sync might not even be a concern. Same if there are parts where the rest of the band drops out in places where the modular comes in.
I've used stompboxes that have tap tempo with limited success when playing bass or guitar. My approach was to give the tap tempo button a few taps every several measures or so, to try to keep the modulation in sync as the drummer's tempo fluctuated. Definitely requires some practice. I can imagine tapping a button with your finger might be a bit easier to coordinate than repeatedly tapping with your foot while playing guitar.
Ideally, it seems like the easiest, most effective situation would be if everyone in the band is capable of playing along to the same (click) master clock as the modular. But, I'd still be interested to hear some others' ways of doing it.