Mail signatures--assigning to specific users
No. This isn't possible with a standard install of OSX Mail.app.

What you can do, as I am sure you are already well aware of, is set up multiple signatures, and manually select the signature that you desire to use. What you can do in the case of wanting no signature is set up a "signature" labeled "Empty", and then put nothing in there. Then have that set to default, or the other way around.