Hmm, so we may as well gather information on what people would like to see in such a thing... I might try and make this. Shouldn't be too difficult.
If so, I'd obviously want to support replays for all four engines (Lemmix, NeoLemmix, Lemmini, SuperLemmini - though does regular Lemmini even have them?). I don't know about Lemmini vs SuperLemmini, but a Lemmix replay can easily be told apart from a NeoLemmix replay just by examining the file - there's a 3-character text string in the header which is "LRB" in traditional Lemmix but "NEO" in NeoLemmix.
For NeoLemmix, it could also be double-checked that the replay is indeed for the level in question (apart from custom standalone levels / DAT level packs, but it can be done for Flexi-based players). This isn't possible for traditional Lemmix, however; though it can be detected which mechanics set was used.
For that which it's possible to do so, I'll probably also include automatically-calculated challenge data. At the very least, any challenge based solely on skill counts can be done (minimum skills, skills you can't live without, etc).
EDIT: I've started working on this. Not going to give a "release date" yet, as I obviously have to do it from scratch, and with that kind of project, I'm the kind of person who could have it done within a day or could take forever.