As the title says. Perhaps a post filter?
There's no way to do that currently, but as you suggest a post-filter can be added, in the same family as the alreadyExists, inMd5List, and downloaded post-filters.
Just added the grabber:favorited filter (you'll have to negate it with - for your use case).
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Just added the
grabber:favoritedfilter (you'll have to negate it with-for your use case).