One other connection here: I’d like to offer a shout-out to @DaveGifford, whose “Toggle” solution, years ago, was my first way to dynamically hide/unhide various view template elements from the sidebar.
Unfortunately, that solution relied on a manual setup for each such element. So that if I added a custom “comments” view template to my wiki, it wouldn’t be part of the “Toggle!” sidebar until I went and hard-coded it in as a thing that could toggle on and off.
And that solution — helpful as it was! — did actually modify the template tiddlers by toggling the presence of the $:/tags/ViewTemplate tag. So if I removed and then re-added the tag to the subtitle view template, the tiddler would be a non-shadow… even if the toggling restored exactly its original shadow contents.
So I am curious whether @DaveGifford agrees that what I’ve sketched out here in proof-of-concept is effectively a good successor-solution to Toggle!
(Toggle also toggles the system tags on edit template and sidebar elements. I haven’t implemented tag-overrides for these yet, though the same principles would apply.)