If I had a gun, and could hit the broad side of a barn, I would be tempted to help nature along, even if it would make me feel bloody awful, but I don't, and the little bugger can still get up a tree on three legs way faster than I can, so trapping him is right out, even if I wanted to risk a bite, which I don't.
So I am watching closely today--if I see him again, I'll call Animal Control and see if they have any advice.
However, while watching closely, I saw Lumpy! I lost track of him after the botflies dropped off, but he was on the railing today, looking healthy, identifiable by the two large patches where the fur has come in a different color and in the wrong direction. So evidentally he, at least, survived. I haven't seen any bots lately, I think it may be a seasonal thing.
So that's a good thing, even if I still feel bad for the hurt squirrel.
Edit: The squirrel--who in a fit of terrible un-PCness, I found myself calling "Gimpy"--is back, still dragging his leg around. I called a rehabilitator and left a message to see if they have any advice, but otherwise I guess I'll just keep an eye on it.