the problem will be that any NAT feature of a vApp network will be bound to a specific IP/Mac.
you can certainly do something like this, just by assigning an IP address .... but it's not really something in the design.
you might want to create a NIC for the sake of having an IP assigned via vcloud, then manually disable that NIC inside the guest ... then you should be able to use it between those VMs.
the only problem becomes MAC based parts ... but I think most of the SNAT/DNAT is IP/Range based ... so you should be fine.