already working on a solution: so far, i've found the registry directory for the context menu is here:
Code:
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Directory\shell\
there are two entries; for this specific program: "mplayerc64.play," and "mplayerc64.enqueue"
".play" is the "play with MPC" action, and .enqueue is "add to playlist" action. i can just delete .play right now and be done with it, but that approach would put me in technical debt later on, in case i might want to change the default back to play.
modifying at this scope, the change would apply to any file that already gives the option for "play" or "add to" MPC and does not require individual filetype modifications as you've described
so i've found a partial solution...