I was wondering, if the contents of a .pyui file is JSON, why create a special extension for it? Many editors don't know how to handle files with this extension, but if they had a ".json" extension, they are handled well. I don't think Pythonista's UI module would have trouble with that extension, either.
Update: I see that Pythonista won't let me rename a .pyui file to .json. It changed my application and its UI file to "fubar.json.py" and "fubar.json.pyui". I guess my point is moot if Pythonista won't go along.