Is it possible to create a network offering, which would use a custom virtual appliance instead of the default Debian template for the virtual router?

I'd like to provide a premium network offering, with a commercial security gateway/UTM virtual appliance as network provider. Ie the FortiGate UTM provides VPN, NAT, DNS, DHCP, routing and other network features similar to Virtual Router, but also offers security features like anitispam, virus scanning, deep packet inspection, IPS etc.

I can create a network like this
Internet -> ACS VR-> Fortigate TM VM -> other VM
But not sure how can force all the traffic from the VMs to go via the FortiGate. So having the UTM as a custom network provider would ensure it?

The environment is XenServer 6.2 and CP 4.2.1 and Advanced Networking

Thanks,
Michal