If you were doing it in VLC before, I wouldn't even worry myself about attempting to do it in MPC-HC. You learned (I think?) in this thread that VLC can do full screen. So just carry on doing what you were doing in VLC, but now enjoy it in your new found full-screen knowledge.
I'm going to make an edit here, this is completely a side note and mostly unrelated to this thread, ignore the following:
learnin - you liked winamp right? I liked winamp too. Winamp was fantastic for music. It did playlists, hell it essentially defined playlists. Now the problem was it couldn't do video. Then Winamp 3 came out, and it destroyed everything that everyone loved about winamp (made some big generalizations there). Winamp 3 started the decline of Winamp. Some programmers apparently figured out that winamp 3 was the wrong direction and also built into winamp 2 the ability to play video. This resulted in a great player, but it was never released publicly, well at least not officially, it is out there - but it was winamp 2 with the ability to play video without the insane bloat that everything that carried the winamp name afterwards carried.
Winamp 4 was skipped. Winamp 5 was created, 2+3=5 was the marketing reasoning. 5 created the bloat and just continued the downfall of Winamp.
So my rant is - Winamp 2 + ability to play video is the pinnacle of a media player, but the times and evolution of user interfaces have killed that possibility seemingly forever. If you want a music player, pick one. If you want a video player, pick one. Maybe you can find one that does both, but it is never going to be like it was in the past, that time is gone.