What OS are you using? Why not just setup the audio rendered in MPC-HC to be one that is SPDIF, and modify the AC3/DTS filter settings to output to SPDIF. No need for AC3Filter (or SPDIFer). Non-AC3/DTS audio should be downmixed to stereo and output as PCM.
I currently do this in Win7, and it works fine.
Of course, this assumes that your audio drivers install a Digital Output device into things.