This isn't exactly a linux based solution but I've setup a similar system. I purchased two RS-232 serial-to-ethernet adapters (small device with DB-9 serial on one side, and RJ45 ethernet on the other) for remote monitoring of a serial device. I had a router onsite configured with a VPN tunnel back to our main office where I had my monitoring workstation. I connected one of the adapters to my serial device and the other to my workstation at the main office, configured them for "tunnel" mode, and through TCP/IP all the serial data was transfered from the remote site to the main office workstation. This solution worked really well and is still working today for monitoring and configuration of said serial device.