Oh, this tag suggestion, was just for example. More likely gambit! is:trash (or :trashed) This would be best. Since as you say, not require Tags extension and not require almost anything at all. Just link to start the search with gambit would be nice. Even more gabit's as we know can be mixed. So is:trash tag:support. When trash gambit is used, in search results should appear additional button. REMOVE ALL Filtered (Founded) .
So we can purge at once whole Trash. Or just part of it. From specific user, tags etc. As you know gambits are mighty.
Or removed individually from trash (forever!). If @Toby didn't want trashed discussion appear for admins on normal lists, trashed gambit would keep them still available and not offending in normal browsing of discussion lists.