I wasn't going to touch the actual schema. Basically I was going to try and figure out the ldap query that zimbra does, and mimic it. Still dangerous of course, as I don't know if it is doing anything else.
I have since found a much better way to do it, which makes me a LOT more comfortable.
Code:
[zimbra@zimbra log]$ zmprov ma artimus@domain.tld zimbraPrefMailSignatureEnabled TRUE zimbraPrefMailSignatureStyle internet zimbraPrefMailSignature "this is my signature"