Easy to do with a script:
on *:INPUT:*:if (/* iswm $1-) set %lastbind $1-
alias lastbind return %lastbind
If you really need it, the above can do it.
Typing
//echo -a $lastbind would produce a
QuineIf you wanted extra parsing of specifics (like separating command name from args, making .show and .builtin props for "." and "!" prefix chars respective) you could script this on as well. You could probably do a better job of anything mIRC did builtin, so you have more power/flexibility scripting this anyway.
Given that it is essentially a one-liner (with small identifier alias for convenience), I don't see why it needs to be builtin. Users who really need this (I've never ever needed this kind of behaviour myself, and can't think of any situations where I would) can already add it.