Hi and sorry for the belated reply, I saw this thread when it was opened but completely lost track of it.
It is perfectly possible to build a browser-based UI for Bela. We are in fact wrapping something up at the moment so that users can use p5.js and other frameworks ( like Nexus UI) to build their own interfaces. There are some decisions to be taken before we release this, but it is almost ready.
In case you want to give it a try, you can use a WebSocket connection between Bela and the browser in a similar fashion to how the Scope works.
The Scope uses a web server library called seasocks (https://github.com/mattgodbolt/seasocks) with WebSocket capability. You can take a look at the source code to get an idea of how this works from the server side (https://github.com/BelaPlatform/Bela/blob/master/core/scope_ws.cpp). Your browser-based UI (the client) would have to listen on the same address and port your server has been stablished.