This can be done via scripts already. Adding such a feature to Mafia would require a considerable extension to how kmails are handled, I think.
EDIT: maybe it wouldn't need that much of a change, actually. It's the first time I look at the Buffbot Manager code, and maybe it wouldn't be that complicated to add.
Of course, it's still possible to do with a script, but that might be incompatible with running in buffbot mode.