Olive "I mean that QubesOS is heavy and includes many VM's inside of it, and so to get that working on virtualbox must surely be really a lot harder than getting GraphineOS to work in VM"
This is a confusing statement. QubesOS is a linux distro that runs virtual machines using Xen-based virtualization.
Using GOS in a VM has no practical application, there is no purpose in it. It's just an android fork and most changes to it don't effect actual user experience.