Currently not directly in a single script. But you could do the workaround I outlined above of first running a script manually on the unmatched files in the left pane. If you have a script like:
$if($inmulti(%genre%,rock),$set(_is_rock,1))
you could run this manually (via context menu → run script) on the files on the left (you might need to drag the files to the left first). Ideally set this script to be deactivated so it does not run automatically.
Then in your regular tagger script you could check for $if(%_is_rock%,...) and act on that.