Cisco IronPort C-series does support VLAN interfaces so you might be able to accomplish what you need by using VLAN's. From the cli type #[]etherconfig and follow the prompts. I suggest you test offline with a lab unit before attempting anything like this in a production environment.
from the product manual
VLANs are virtual local area networks bound to physical data ports. You can configure VLANs to increase the number of networks the Cisco IronPort appliance can connect to beyond the number of physical interfaces included. For example, an IronPort C6x appliance has three interfaces: Data 1, Data 2, and Management. VLANs allow more networks to be defined on separate “ports” on existing listeners. (See Appendix B, “Accessing the Appliance” for more information.) You can configure multiple VLANs on any physical network port. Figure 7-1 provides an example of configuring several VLANs on the Data 2 interface.
