I have often thought of a different approach. Rather than a web verson of Frescobaldi I would like to see a remote server - similar to Lilybin - with an API that can be called from within the normal Frescobaldi program.
All fancy editing and viewing features of Frescobaldi would already be there, only the rendering to pdf/svg/midi would be done by a remote server online.
It's already possible to choose between viewing the lilypond documentation online or locally and offline inside Frescobaldi. Wouldn't it be nice to also be able to choose between a locally installed complete Lilypond engine or a remote online Lilypond engine?
When Lilybin was still alive I used little script that was able to call lilybin from the commandline and I was able to send a lilypond file and getting a PDF and/or MIDI file back - without using the Lilybin website, and without having LilyPond installed locally.
This makes me believe it can be done.
Just an idea.