If you can't use bridging mode for your virtual machine's NICs in your configuration, you might try setting up a separate private network and set up multicast routing between the pivate and the public network, presumably including NAT for outbound private traffic.
A possible solution might include host-only networking (this would give you a virtual NIC in your host operating system not connected to anything else but the virtual network) and the appropriate multicast routing / NAT setup at your VM host.
I don't think this is a good task for a Windows machine. If your Host must be Windows and you are not able to either get even a single further IP address for a Linux guest VM or a separate appliance performing this job or move the public IP address your host uses to the aforementioned Linux guest, you are probably stuck here.