Would anyone know of a virtual machine software that is able to boot off WUBI-created root.disk's?
Failing that, would anyone know of a virtual machine software whose virtual disks are also natively bootable so one could alternate virtual and native boots off the same virtual disk?
Reading that two links, I guess that you'll need to create a small image containing a bootloader that points to a second image - the image containing the root-fs (in this case, the image of WUBI). But I don't know if will works smoothly!
But I emphasize: this strikes me as an experimental procedure, used very little and you hardly find a definitive tutorial describing how to proceed.
If you really need to manage multiples virtual machines, just create those VMs into a pen drive and use it inside different OS through virtualbox. Or you can try the procedure described at those links.