IIRC it wouldn't be a problem from the bhyve side, but I hardly doubt Oracle will do anything to make it work on the Virtualbox side.
If you want/have to run legacy VMs on a 'legacy' hypervisor, you might want to take a look at smartOS, which can run bhyve and KVM concurrently without any performance impacts. We are running KVM+bhyve on the same smartOS hosts in production for nearly 2 years now, with KVM VMs heavily declining in numbers because everything halfway modern runs perfectly fine on bhyve, so no need to drag legacy cruft around like an emulated ISA bus or floppy disc controllers... The KVM VMs are usually legacy VMs (i.e. appliance vendor doesn't support bhyve or crufty old 2.x linux based appliances...) or VMs that are bound to be decommissioned in the near future; everything that has been set up over the last ~1.5 years is running on bhyve.