Interesting Effort!!
I Think that Would be Funnier to Introduce in a Text Field the Html and BB Codes for a Fake/Fun Signature and Choose which Usergroups and/or Users to Change their Signature while the Hack is Turned On; They'll Say "What Happened With My Signature??"; They'll see in their User Control Panel that their Signature Still Right but Shows "Wrong" and will ask "What's Going On", and You'll Tell them "Happy Innocents Day"; Could be Nice on Innocent Days, but not something to do on a Regular Day!!
My Best Regards!!