Local face remapping

[JD Smith]

My indent-bars package makes good use of `face-remap-add-relative’ to provide performant depth-based context highlighting via a post-command-hook.  This is very fast and simple: just check the indentation depth, and if it changed, update the buffer-local face remapping list accordingly.  I am now considering a feature which would make this highlighting local in regions based on treesitter block scope.

Is there any means of achieving local face remapping within regions?  Ideally overlays could be used, if they had a `remapping-alist' property, similar to the global or buffer-local value, but applying only within the bounds of the overlay.  Given that this does not exist, is there another way to achieve something similar?