I'm not very familiar with the S8100, but if it has a terminal connection, then you can put it on the network. We have several systems ranging from V4 to V8 that don't support networking. So I purchased a device from Digi called Digi One SP. They cost about $170 each and allow you to connect to the terminal connection via the network. Another device called LANSAT does the same thing but costs more. Basically what happens is when you connect to it via a telnet session (what ASA uses on the network), it will forward everything to its serial port. Kind of works like a bridge from the console port to the network.
The only drawbacks are that you can only have 1 person at a time use it and the local terminal can't be used (only an issue if somebody local to the system uses it). Even with these limitations, they still beat using a modem.
If this sounds like something you want to try, let us know. I'll then write more information regarding the Digi configuration and wiring.