It's an interesting idea, but how well is that supported? Are there any other Qt applications that have already done this? I have currently no plans to work on this, but I'd welcome any effort to get that to work.
You can check out these resources:
https://github.com/msorvig/qt-webassembly-examples
http://vps2.etotheipiplusone.com:30176/redmine/projects/emscripten-qt/wiki/Demos
Amongst others.