That's odd, because I've done this on my Z8 and it worked. I also built a PC based PDA, including my own cables, and it works. (descriptions are in this forum someplace.
I just hope you didn't confuse the EPS diag port with the ECU diag port.
The 500 uses a Bosch ECU, the 800 uses a Delphi ECU, but they should both flash codes on demand.