So any word on this? This feels like it might be kind of a dealbreaker for anyone trying to use JSON for anything non-Spine-related @_@;

No word from the core team yet I'm afraid. We'll likely have to use a custom extension for .json files. We already do for imported assets, .spjson, and I guess I'll use that for files to be imported as well. It's not great, as that will break existing projects :/

Regrettable, but better than nothing.

One way you could go about it is adding it as an export option in the editor - like, an "Export for Godot" option that simply changes the export extension. You could even provide a batch file for converting existing files (which, technically, is something anybody with some command-line expertise could probably do, but still)

I tried to find docs on how to add settings for a plugin but came up with nothing. Do you happen to have a link? Link to sources would also be good enough. Also, it seems the plugin's importer is only queried once for its supported extensions on load, so I wonder if this setup would work.

Oop - I was suggesting a tweak for the Spine editor, my bad. I'm afraid I'm not familiar with Godot's inner workings.

Mario wrote

No word from the core team yet I'm afraid. We'll likely have to use a custom extension for .json files. We already do for imported assets, .spjson, and I guess I'll use that for files to be imported as well. It's not great, as that will break existing projects :/

It's a shame Godot doesn't allow this :/, if there are no other alternatives then I guess introducing a breaking change this early wouldn't be that bad since the runtime released just recently and most people's projects are probably still in early stages.