I've known a capella singers to join the Open Music. And even be well received, though it's hard for instrumentalists to join in with them, other than simple percussion. There've been some very lovely opportunities for vocal harmony, though. (I always enjoy it when mplsfish sings LCohen's Hallelujah.)

I'm sorry to hear about the trouble with your ankles. I got a small open circle going in the bar for a while on Sunday, and another singer would have been welcome.