I have found IODD 2531 (there are likely others) to be super-useful for that sort of a thing.
It presents an ISO (using the rocker on the side as a selector) from its HDD as a bootable CDROM target, and still exposes the HDD/SSD as a separate target.
So, it is possible to boot what is an RO OS image, and use the HDD/SSD target as an overlay, but because it is not flash memory, the wear issue is far less of a problem.
And, since it is presenting ISO images as boot targets, it is possible to (relatively) easily test upgrades while keeping the overlays separate.
