I put together the start of an xdg-desktop-portal interface here: https://github.com/snapcore/snapd/compare/master...jhenstridge:xdg-desktop-portal-interface
Unfortunately, I ran into some problems getting the document portal bind mount working. Like I said in that forum thread, I don’t think it is reasonable to try and reimplement any of the xdg-desktop-portal services: toolkits are expecting the full suite of services to be available, and matching the semantics of things like the document portal would be non-trivial.
I put together some notes on how we could go about integrating the code here: