since when you are watching a replay, Stunts regenerates the whole physic happenings, collisions, acceleration, jumps, etc, everything, it's impossible to do a separate RPL viewer/editor without the sources of the original stunts physic engine.
if we'd have the sources it'd be not that hard to code an optimized replay searcher/writer, so let's celebrate we dont have the sources.
well you have at least the possibility to write a wrapper that hooks into the replay-engine when stunts is loaded to memory and directly passes commands at the engine and fetches the result. of course this would be not very nice but I think it'll work.
second thing would be to reverse-engineer the whole program, first use a decompiler (if I remember correctly stunts was written in Microsoft C 5) and then translate the remaining asm-code per hand - time-consuming but a chance to get the code
ok, you have the code. so what to do now?
replay editor? hex editor is good enough (and still do not make good results)
some tester of bruteforce replays? ha ha
the same but with heuristics? hmm.. these should be really good heuristics to complete a run within a month.
some machine learning? i can imagine to train some situations and then apply it. still it would discard some of the best tricks. but really usable with OWOOT rules! the note: that training has to be huge because of many entrance speeds and many track's pieces combinations to learn.
ok so with the whole engine we could have a machine that makes nearly perfect replays with owoot rules. as long as it does not making money it's good only for ruin a game and probably master thesis.
thinking again about the tester with heuristic.. probably, but I don't have any clue.