Shoot, as soon as I hit reply some neurons lit up and now I remember I was actually able to enable nested virtualization in QubesOS, and the relevant options in the VirtualBox preferences inside a qube became enabled once I did that, but whenever I tried booting any VM the whole system hanged. The same system and BIOS settings worked in Ubuntu to boot a nested VM in VirtualBox, so I think I had the BIOS settings correct. Anyhow, it seemed like a dead-end, so I abandoned it.
>>flashb+(OP)
I'll have to look at it more. I mostly use AMD systems these days, which don't support nested virt in Xen, as I understand it, but it looks like it should work on Intel.