From one perspective, all my side-gigs are for equity: I own them 100%, and I get no salary.

But assuming you mean would I work on someone else's side-gig for a slice of equity ... maybe?

A lot would depend on the situation, but I can see that if it was a project I was interested in, and someone else was working on it in their spare time, I could maybe be persuaded to join in for a decent slice of resulting entity.

I guess it's pretty unlikely that the right circumstances would arise, but it's definitely possible I'd consider it.

I did that once. Made a thousand dollars for my efforts when it closed down.

As long as the expectations are not too serious and it is not a crackpot idea, why not? I have a main job so as long it does not interfere with that, all is well.