To answer my own question, I did a some more research after posting and found a code for a Marantz AV1041 pre-amp that does the job on my SR5200. For anyone who's interested it's RC5 16 37. I also found a CCF for the SR7000 that has discretes for the various DSP modes (hall, stadium etc.) that work on the SR5200.
Now if I could only find discrete codes for the Prologic, Prologic II Movie and Prologic II Music modes...
...Andy