GBPVR uses the codecs on your computer to play back files. I'm not too familiar with mplayer, but I think Zoom player basically does the same thing, being a front-end for a bunch of free codecs all tossed together. So if a particular audio or video codec you want to use is on your computer, you should be able to use it in GBPVR, but you'll need to use the GBPVR interface while watching.
It'd be great if GBPVR could use the 1.5x and 2x speed functions in PowerDVD, but the best it (or any other program) can do is use the same audio and video decoders.
I use ZoomPlayer to watch videos from CD. I created a "custom task" in the Config app. I use HIP so it automatically switches what the remote controls and I set ZoomPlayer to close when the video completes and on stop and have it automatically launch on fullscreen. Not ideal but pretty seemless.