Why not a hybrid approach?
Nearly every device, system, or software that I have come across have the ability to do signatures. But why not use a simple ASCII string that would contribute to a match and replace so that way it fills the signature in the right place every time?
Like, signature on any device could be like ///===EMAILSIG===/// or something then have the server find it by having a customizable string field to find and match. We would be able to circulate this string throughout the organization easily and even push it out in preassembled installables.