Never thought of that - it’s like 1-2-3 
- nice idea. There has been the occasional question on making a live ISO from your running system - which is a completely different matter.
If it is a requirement to use the graphical installer - this could be a way of reaching the goal.
Still think it would be easier to target the USB directly - optimizing the install for USB - e.g. the graphic drivers which - to be truly portable - must include all open-source drivers - not only the virtual driver