I didn't really think about it much, but it seems mIRC doesn't allow _any_ regex features in the "replace with" part other than \#, so the only thing I could come up with is something like:
$regsub(test,abc,(b|d),\1 $+ $chr(255),%var)
%var = $replace(%var,$chr(255),2)
Since character 255 is the character for the delete key, you can be reasonably sure that will not appear in your text. It's not a good way to do it, but it works.