Can't guarantee this to work, but it should as SIM card usually uses very low rate (kilobits/s) to communicate: add extension between the socket and SIM card, just wire them together over 50m of STP cable.
Get two SIM card extension cables from China, then solder cable between them to make them longer, maybe even add RJ45 connectors so you can easily plug it into existing wiring.
The SIM standard is here: http://www.smartcardsupply.com/Content/ ... andard.htm
Some modems do have "soft SIM" where it's possible to send SIM card commands over AT port, but there is no standard, it's all proprietary. And you would need SIM card reader you relay commands to on the other end...