My receiver is a Denon 3808CI and it does support uncompressed pcm via HDMI. Yes I have a 5.1 system, actually 5.2 and everything works perfectly fine when playing a blu ray movie or dvd.
If I set my speakers in MC to 5.1 and play the blu ray of 3:10 to Yuma (it has an uncompressed PCM 7.1 soundtrack), my receiver will display mcpcm and plays fine. If I play a DTS-HD Master Audio track, my receiver will display DTS-HD Master. So far so good. But 96/24 flac files won't play but normal flac does.
Now if I change my speakers in MC to 2 channels, the PMC 7.1 soundtrack will no longer show up as MCPCM on my reciever but instead default to DPLII Cinema. At that point I can't get it to select MCPCM. However, in this config, the 96/24 files will play.
So I'm not really sure what MC is doing.