The signature Jeff proposes is basically what I had in mind. Formalizing the naming convention, and perhaps putting the string "callbacks" somewhere in the namespace as a safety, would then allow us to track down the stuff to call using "info procs".
On a brief skim I think APM callbacks have a similar signature but that APM ultimately stores the callback in the db. We'd be generalizing that design and moving the storage from the db to Tcl memory.
