First thanks a lot for such wonderful feature!
My issue is about ESXi 6.7.
I installed ESXi 6.7 on VirtualBox 6.0.8 (amd64), then created a 64bit Linux guest on the ESXi.
I could created it without any issue so I thought the Nested virtualization did work and I also confirmed this point through below ESXi command.
[root@vCenter:~] esxcfg-info -w | grep "HV Support"
|----HV Support...............................................3
While when I tried to start the guest I would get error 'This host does not support "AMD RVI" hardware assisted MMU virtualization' and failed.
I did similar things with VMware workstation and ESXi then also created a 64bit LInux guest on the ESXi, and it worked well.
So it seems VirtualBox could not expose AMD-RVI to the ESXi guest, right?