Whether its technically possible to do so or not is out of my realm of knowledge - however, currently the system seems to take into account current user profile data for certain things (such as your post counts, your location & affiliations) and I assume that since the signature line is part of your profile, it is something that can't be changed as you want. But that's my guess...
Once in a while you get shown the light in the strangest of faces if you look at them right.
Agreed. The current version is fine. I use my signature for links to things like my flight memory and notification of upcoming DOs I'm participating in or arranging. Wouldn't want old ones hanging around.
Another suggestion re: signatures: I understand the reason behind the 250-character limit (ugh...some of the sigs on other fora are unbearable...), but is it possible to make the 250-character limit not include BBCode? I had a fairly short signature I wanted to add, but I couldn't make it smaller using the size command or link to the full URL I wanted to link to because of the limit. As long as the BBCode doesn't result in a visibly longer and more obnoxious signature, what can it hurt?