[qqwref]

Would it be possible to not delete "used" notes, but simply move them to another array? That might make it easier to move around in a file. (Or actually... how about just using a pointer in the note array, and then incrementing the pointer rather than deleting notes? Can that be done?)

As far as the idea of clicking on a time and playing the replay from then, how about this: remove all arrows and replay hits before that frame, then essentially start playing only the part of the chart from there to the end. You would have a ~30 frame empty spot at the start before the notes start to appear, and with some clever mp3 manipulation it might be possible to get it to display on sync. I'm not a flash expert, but from your description of the arrow spawning engine it seems like it could work.