Here's something i came up with for hacked replays that may or may not be doable
slowmotion.lua- divides all velocity values of all joints (or one player's joints) by a given value on the frame the command is inputted, and gravity by the same amount. Automatically saves the replay to a specified output name much like nuthug's recordframes script.
and i guess you should be able to use negative values to reverse the effect
I made a slowmotion effect by doing this manually in notepad but it took ages. This could maybe be something that could create some sick replays if anyone is up to try