Right now it looks like the answer is "no". I have a GMail account set up in the GMail app, and 3 other email accounts in the Email app. GMail is no problem, but the other 3 in Email all use the same signature. Which blows. I set a fairly generic signature but it's far from optimal.
As a former BlackBerry user I really miss the app called Autotext. It would convert a pre-defined letter/number combination to whatever you wanted. So when I typed "bbba" at the bottom of an email it would become my signature A. Likewise, "bbbg" converted to my GMail signature. I used "bbbp" for a personal signature, no business info, that was 4 lines of contact info under my standard closing (Regards, Bob). The trick was to use a letter combination that you wouldn't normally type, otherwise Autotext would change it and you'd have to backspace to undo the change. Very simple fix, but if you didn't notice that Autotext had done it's thing you would have words inserted in seemingly nonsensical fashion. The other great thing about Autotext is the ability of a user to program in commonly used words, so typing "ty" in an email could be translated to "they're".
Failing the inclusion of unique signatures for each account, or selectable signatures on a per-message basis (not optimal, but better than no choices), Autotext would be a wonderful solution.