What's the issue with using External Networks in vCD? - this is exactly what it is designed to do.
Define a VLAN-backed PG in vCenter which spans to the physical switch ports you need for your physical hosts, then provision VMs into the same network in vCD by creating adding the external network to a VDC and creating an Org VDC network backed by the same PG.
The only annoyance in this is the requirement to defined IP settings (at least a single IP address, mask and gateway) when defining the external network in vCD - but these can be dummy values (or you can simply allocate a single unused real address to the pool.
We do this quite commonly when customers have physical equipment in our Datacenters shared with vCloud Director hosted VMs - unless I've misunderstood what you are trying to achieve?
