You want an "open jam", not an open mic. Even open mics can give a bassist opportunities. Learn some of the songs you heard the acoustic acts playing, and then when you have it down, ask if they want some bass. That's how I hooked up with one of the best singer/songwriters in the northeast; I bought his CD, learned every song on it (the CD had no bass on it, just one guitar and vocals), approached him one night asking if he'd like some backup on a couple of original (his) blues tunes. I nailed it, and I've done hundreds of paying shows with him since. Even on nights when there are three bassists in the house I get the lion's share of bass duty, by request of the performers. I am an acoustic singer/songwriter, but at this open mic I don't even play my own set ... I'm on stage most of the night on bass. And don't be surprised if you become a very "in demand" bass player.
