Hello There is a cydia tweak called SMS Signature which allows there to be signature in the messages app. It has not been updated to work on iOS 6 . I have tried contacting him but have gotten no response. It is a very very simple tweak I have looked at it myself I just don't know anything about objective c. I have tried but I am waiting. Basically just download it, look at it and see what needs to be updated and send me it updated. Please and thank you. Reply if you can help me.