FWIW… I just tried this out today. It’s very promising but has a few bugs.
Better use Chrome. I first tried in Firefox and objects weren’t saved/propagated correctly. Seemed better in Chrome.
Some wonky things with drag/drop positioning with the smaller canvases.
Buttons seem to fire multiple times for one click (need to verify that again).
On the plus side – it’s really easy to drop controls on the canvas and set their properties, and then you can load
192.168.x.yyy:8080/preview on a phone (for some reason, that didn’t work the first time but it was ok later…) and the controls do send the messages out. So for the basic stated purpose, it’s on the right track