Currently, it can't.
But what about a different naming scheme?
First you rename the files with e.g. %artist% - %title__
(the double underscore at the end)
and then, straight away you rename the files again with ordinary %artist% - %title%
All the files that could not be renamed keep the double underscore and you can filter for them with %_filename% HAS __
IMHO it is an even better solution as otherwise, with the saved list of files, you would have to load each of these files individually or filter for them one after the other.
The underscore is something where MP3tag takes over the search - probably much faster than an human can do - and you have the list right there.