My idea might sound supit, yet if you're really desperate, you should use it
:
At the end of a MPmission (I don't know how it works, but it should be like a conventional mission) set the score of each player to '0'.
There are a few commands for score editing. Try them.
Check sintax, first
score (get players score)
addscore (the command is self-explanatory ;D )
At the end of the mission, foreach player do this:
_scr=score _player
_player addscore -(_scr)
Does it work? Hum...