[igotrhythm]

Regarding the actual score being kept track of in game, I went and knocked out some pseudocode regarding how this might be accomplished. The key hurdle here, that of keeping track of max combo, is resolved by the fact that multiplayer already is able to keep a running max combo count during the song, which means that the game engine can do it. (Boo out on purpose during an MP game and watch it yourself.)

The other part is how much each "step" is worth:
Perfect - 550
Good - 275
Average - 55
Miss - -310
Boo - -20

With this in mind, the only thing left to do is program the running max combo bonus something like this (I learned C++, so you might need different notation):

Score = 0
currentCombo = 0
maxCombo = 0
...
If stepGrade == (Perfect OR Good OR Average) then currentCombo++
If stepGrade == Miss then currentCombo = 0
...
If currentCombo > maxCombo then maxCombo = currentCombo and Score + 1000 = Score

That is, each time the max combo is increased (by virtue of current combo increasing to the point that it exceeds the previous max combo), the score goes up by 1000 in addition to the number of points achieved for the last step's grade. I think this method is far easier than running the end-of-song score calculation each time the grade totals change.