see Import 3D Asset without servers
If you are really great at javascript you may be able to write a userscript and extend the designer front end, but all model changes would not be saved since you’d need to re-upload the model with the changed Geometry/UV map. So, you’d need some export functionality too. Your web client has all the data for the model/uv map in order to display it, so it doesn’t sound impossible.
For example, I wrote a very simple script to increase the zoom rate when you hold shift here: https://monkeyguts.com/code.php?nav=descr&id=613
You need tampermonkey or a similar extension to get that to work. Writing a 3d modeler and uv editor in javascript sounds like quite a bit of work.
Kinda related
Dave beat me while writing… 