You could convert the XML into JSON and then use it. Part of the reason why the REST API only uses XML is that it actually uses XML internally in many cases.
It would be useful if the REST API could return JSON and have the conversion done server side, but that is not an option currently.
The WebUI is written in Node.js so I would think it would want to use JSON as well, but I have no idea how that works under the hood.
Also, are you aware of the WebUI? It might replace your need to build an external mini webapp.