I don't do flac or mpc, but would something like this
work for what you have in mind:
Sometimes when I watch videos with MPC I find that I want to add some more videos to the playlist so that’ll they run after the video I’m watching. This can be done from the program, but you’ll have to muck about with opening the playlist and browsing through the programs little “Browse…” feature. It’d be very convenient to be able to add videos, songs, etc, to the playlist by just right clicking on them in Explorer (Like you’re able to with Winamp). Well, this is possible thanks to the command line switches MPC has.
In Explorer goto Tools -> Folder Options and select the File Types tab. Select the file type you want to enable this for (for example .avi) and press Advanced. Press New. As Action type in:
Enqueue in MPC
and as Application type in:
"C:\WINDOWS\mplayerc.exe" /play /add "%1"
That is, if that’s where your Media Player Classic installation is located. If you installed to another directory, you need to adjust the above line to fit your installation. When done editing, press OK three times to get out of the menus.
That’s it! Now you should be able to right-click on any .avi file and select “Enqueue in MPC” and the video should be added to MPC’s playlist. I suggest you also add this Action to the following file types: .avi .mp4 .mpg .m2v, since those are all fairly common for video files.
The text implies that Winamp will let you add to the playlist from Explorer too. If your favorite player has a command line interface with appropriate switches then you could probably modify this process.