maybe a generic widget allowing to select the "directory" to show in the branches view, For example, in this DGtal repository, it would propose a dropbox with the following items:

• /ref: show all the branches
• /ref/heads show only branched under this "path"
• /ref/pull id.
• /ref/tags id.

You're absolutely right that this is a different concern than T2459, because even when we will fix that we will not rewrite history.
On the other hand the heuristic that the loader will us is probably related to what you want here: you'll want to filter out by default the same refs that the (new) loader will filter out, and have an option to unfilter them (mostly useful for old snapshots).
If we have that, will you still want the fine-grained filtering that you mentioned above? I'm inclined to think it would be overkill. YMMV.

I don't mind a "quick" solution that fixes the problem right now, and revisit it once we have T2459 fixed. Not sure I agree with the statement that my proposal in the comment is "fine-grained" though. I mean I suggest nothing fancy, just a few entries in a dropbox which content is built from the "directories" of the branch names.