From the VNC FAQ:
"At present we just use TCP/IP, because it's convenient, ubiquitous, and easy to route. This means that you can use VNC over anything which supports TCP/IP, so using it over a modem is just the same as any other network, once you have Dial-Up Networking set up. If you need to communicate directly between two machines without going via the internet/intranet, then set up a remote access server on one and dial in from the other."
In other words, set up your null modem cables and run as if you were dialing up; any TCP/IP based solution should then work.
As I understand it, this is essentially how PCA does it's serial connections it just hides the setup.
Another alternative would be to use a KVM-over-Cat5 solution, the kind that has a transceiver at each end & use cheap CAT-5 for cabling instead of proprietary KVM cables (allows for far longer runs as well).