In my last band, the only one I ever led, we’d start by locking in a groove, something one of the players would (usually spontaneously) come up with, usually 4, 8, or 12 bars. Nothing too complex most time, although my bass player had a thing for weird time signatures.
We’d try to get super tight on just this loop until someone came up with a complementary set of changes/rhythm, then we’d play until we locked that in too. We had a few times we would record this live, and it would become the backbone of a future song.
We’d usually of repeat this until we were board, which usually happened around 20-30 minute mark, or someone shouted our safe word for too long solos, Freebird.
(Warming: this would sometimes devolve into us playing weird covers of Freebird. Trust me you, do not need a reggae or bluegrass Freebird cover.)
This is not exactly like everyone warming up to scales/crazy licks/pyrotechnic playing like you envisioned, but I think it helped us lock in tighter in our “real” songs.
Sorry haven’t played in 15 years, and it sucks. Be the bluegrass/reggae you want to see in the world!