So basically what you're looking for is some sampler capability from mpt... What sort though? Do you want beatmachine style sampling, where the sample is sliced into a whole bunch of little "sections", each of which can be individually played back? Or do you want a slightly more loopier system, where you can "snap" the cues to the beats? That is, you can tell mpt where each beat of the bar (or subbeat, or whatever) is, and it'll make sure that no matter what tempo the sample is played back, those cues are hit at the right time?
It sounds like you want to turn the sampler-ness of mpt into something a bit more serious than just playback, and I'm all for it. Another idea: why not integrate it with instruments? I'm guessing that this'll be mostly useful for pre-recorded beats/melodic phrases, which'll tend to be played at about the same pitch, right? So, assuming the beatmachine sampling above, why not assign a different note to each cue? For example, you could have normal notes C4..B6, then have cue 1 triggered by C6, cue 2 by C#6, and so on. That'd be a bit easier than using effects, and it's the way that most samplers seem to work, which would make it more "plug and play" friendly...
While you're waiting for this feature to be implemented, it might be worthwhile trawling the net for some sampler vstis, just to get a feel of what can be done, and how. I personally use Kontakt, which offers a whole lot more than I've outlined above, but there's a whole raft of other samplers available out there.
