I just finished a half duplex serial asynchronous link from a simple PIC circuit to the RCX. It is able to receive codes from the RCX (or Lego RCX remote control) store them in RAM & EEPROM and transmit any IR op-code to the RCX in turn. Included in the firmware is a routine that takes any opcode(s) and expand it to the proper IR packet for transmission to the RCX. I've used the UIRT circuit with a few modifications for the hardware. The UIRT includes a serial port programmer for the PIC16F84A so you won't need a separate programmer.
1) connect a jumper from MCLR (PIC16F84A pin 4) to the V+ (+ end of the 100uf capacitor). This keeps the PIC from resetting.
2)Build a 5V power supply. I used a 7805, a 0.1uf cap. and a 9V battery connected across the 100uf capac