Implement top quoting support for signatures. There's a new config option to include signatures before quoted text, and a new entry in the composer's edit menu to prepend the current signature. Identity switching tries to remove the signature, if it finds it anywhere in the body, which has some risk, but I guess that's not a real issue. In top quote mode the signatures are added without -- seperator, as seems to be the convention.
(Kolab Issue 1385)