Depending on your abilities, could build up a single chip processor like a PIC with RS-485 serial input, and the PIC would received the command, determine if it was the one on the RS-485 (assuming more than two nodes), and turn on, off or flash the LED. Kind of an overkill, but you need something to interpet what's coming across the RS-485 and then act. If there is only the 2 nodes on the RS-485, might be able to have a flip/flop toggle, but if things got out of synch, not good. If there is only that one thing to control on the pair of wires, why not just hook the pair to an output with a resistor and the LED on the other end? IS there a spare pair of wires going there?