Gardner:
Implementing this hack went just fine, but I ran into a problem during my testing. When I delete a signature that was used for a post, the post that used the signature disappears. It is no longer visable in FLAT mode. I suspect this is because it is trying to grab the signature, but it no longer exists. I would recommend putting in a bit of error-checking that if it can't locate the signature, it "defaults" to the Default. That would prevent the post from disappearing.
If you have questions on what I mean, feel free to contact me.