I've recently setup a configuration using an 8 port Comtrol modem, Linux, ser2net, and virtual
machines running Windows using free (but proprietary) virtual COM port software from HWGroup to
allow each one of the Windows machines to have one of the modems virtually mapped to a COM port.
The settings were a bit finicky but eventually I did find ones that worked reliably. I had to turn NVT on but turn the NVT filter off. Apparently ser2net does not encode FF's to the remote side of the connection during data communication which that feature expects.
I haven't found any mention of using these two pieces of software together so I just wanted to mention that they work and that its a possible new and powerful use for ser2net. Congratulations on interoperating with something that it sounds like you never intended to.