Wow this is really cool !

This pull request is not to be merged as is,

I've converted the PR as draft to reflect this

but because there tons of libraries that can be now be employed alongside the game engine.

Yes, that's the main selling point for using Python instead of GDscript ^^
I don't much about Python ecosystem on Raspberry, is it easy to pip install binary dependencies ?

the broader picture is detailed in a tutorial here

This is a really nice tutorial, do you mind if I add it to the README ? (it could help people trying to use pip on top of using raspberry)

What would be the best way to integrate those changes? Create a dedicated platform, e.g. x11-64-arm (maybe with an "-experimental" flag)?

Adding a new platform seems like the way to go.

However I'm not sure how we could test the platform on the CI. Maybe we could use an emulator, but given Raspberry support for Godot is not official, we may endup with fragile code (especially when Godot 4 will be released) and I won't have any experience to support this.

I think the simplest thing to do would be to add the platform, but with no guarantees whatsoever (so no CI is run against it, people using Raspberry could submit PR to fix issues but no effort would be done to check for changes compatibility)

Another solution would be for you (or anybody interested in supporting Raspberry) to fork Godot-Python and add the new platform from there (typically using git subtree to simplify synchronization with the upstream Godot-Python).

Again, Godot 4 brings a complete breakage of the GDNative API so I can't guarantee how much the Godot-Python codebase will change to support this (in theory the platform stuff should not change that much given it's mostly the interface with Godot that is modified, but who knows ! ^^).