So, it looks like the biggest issue isn't being able to build for Raspbian, but that the Godot Engine and games run too slowly. There are methods for building native apps (less likely to work) that should be slightly faster; and methods to bundle in Electron (more likely to work, but probably slower). The question is whether one of both of these would produce a game that runs reasonably well on a Pi.

I'll try make a build for Raspbian this weekend. Worth checking - maybe the info I'm going on is outdated.