Check out Jeff Miller's "Acts of Daring" in this week's LA Times Calendar section -- he spent a couple weeks visiting a bunch of area nights and writing up his impressions. Definitely a familiar ring to a lot of these nights, although this one caught my eye...
[At Suzy's in Hermosa Beach...] Regardless of whether he knows the performer, the host sings harmonies from his seat.
Just speaking for myself, but I'd probably want to beat the guy senseless. Just ask first, you know? It's OK if, for a few minutes, attention is not on you. This is what I get for posting before I've had coffee. I'm sure he's a perfectly nice guy. But still, ask.