ReadyNAS OS 6.9.3 | Run on existing x86 (4.2) #notsupported

I've been thinking about installing a custom OS on my Ultra 6 for a while now (the 4.2 OS is getting outdated and I like encryption). While trying to figure out how to avoid the system from overwriting my disks during boot without breaking my options of a rollback, I came across this post, and it's definitely something I want to try.

Question though: Since this most likely messes with the internal USB/default image, is there a recommended way to backup a clean 4.2 image, so that I could return to the stock image? Or will I be stuck on 6.x?

Hypothetically, if I remove all my disks, insert a single empty disk (or no disk at all), and let the Ultra reset to factory default, could I make a backup of that state, that will allow me to return to it (possibly to the point where Netgear would actually support it again).
Knowing that I have that option would help me experiment more freely.

I have a VGA cable on the system, so I am able to get into the system at boot.