Hi Norman,
Based upon my current understanding of Genode, which is still minimal but growing steadily, as you move to get VirtualBox running well, it seems that for the networking to be viable in the VB instance, you might want to support network-bridging as well as NAT (masquerading IP's) in a similar way that VB currently allows and also how XEN seems to work in that each VM instance can be bridged to the host with a "real" IP of it's own on he same subnet as the host, or to possible use the VB built in NAT support VM instances on another subnet but all channeled through the host network adapter. Typically, it seems that VB sets up a TAP/TUN device (virtual network card) on the host and then allows for the various network setups like NAT, bridged, host-only, etc...
As I was not sure if these questions had been investigated, I thought that perhaps Open vSwitch ( ie. a software hub basically) might be easily implemented in Genode and serve as the networking center for VirtualBox in a similar way that XEN seems to do it (
http://wiki.xen.org/wiki/Xen_Networking).
This allows VB under Genode to support many VMM's concurrently.
You have probably already thought of these things, but I thought that I would mention them as the question arose in my mind as well.
Just some thoughts that I had since my goal is to try and set up NOVA-Genode-VirtualBox in a complete Type-1 Hypervisor that could be competitive to XEN while being much more secure, stable, and address the shortcomings found in that hypervisor which is very code bloated and heavy. The NOVA-Genode-VirtualBox approach should require much lest LOC and in general be better given the wonderful design that you and your team have developed from the inception.