It's even easier now - no cross flashing required

• wine is still required for MediaCodeSpeedEdit (Zip/EXE, 178 KB) to modify the firmware
  
• DVRFlash is still needed to flash the firmware

Pioneer released a newer firmware (1.09) (Japanese site, in English).
Pick up the DVR-216 (Zip/EXE, 1.1 MB) firmware you need (DVR-216, DVR-216SV, DVR-216SV are the same drive/firmware).

There are other variations on this drive, and there are different firmwares for them.

Firmware 1.06 is available at their DVD Asia site

I checked the MediaCodeSpeedEdit help, and it knows about the DVR-216, DVR-216D and DVR-216L.
All I needed to do is use it to modify the downloaded firmware and use DVRFlash to flash it.

I then verified with DVRFlash that the drive was indeed RPC1