A Hubitat app can serve HTML (and similar) content from a defined "mapping"/endpoint with OAuth authentication--or use these endpoints to receive pretty much any data between the app and something else (and you can send via the regular HTTP methods available to apps). Here is a minimal example a community member made that I searched for:
If you remember how SmartTiles (precursor to ActionTiles) worked, this is basically how I remember that working, which sounds vageuly similar to what you might want. But that product is long gone. 
