Disclaimer (to everyone): Please oh please bear in mind that all things UI are speculation at the moment. Especially from me, since all of it is OJ's call. ty :-) . With that in mind:
My current thoughts are that we'll have an SVG/javascript interface, maybe a Python (or whatever's convenient) CGI script, and the AVR GCC toolchain set up - that the JS and Python will, between them, somehow, generate C code - and that this C code will then be compiled, and the binaries served to the user. I imagine we'll have our code up on github, for collaborative purposes if nothing else. And *if* I'm right about all these things, I can't see you having any difficulty running the UI on a local server, with a modified backend script that also flashes your Teensy :-) .