Sometimes I talk to myself. Why just the other day, I remarked:
Chris, I’d really like to apply two different sets of effects to a single GarageBand drum loop—one effect that’s used for the bulk of the loop, and another that splashes a mess o’ reverb on the very last whack. How would I do that?
Well, I replied, I’d suggest that you insert the playhead just before that last whack, and choose Edit: Split to separate the last whack from the rest of the loop.
Then create a new Real Instrument track, move the whack to that track, move it around so it fits seamlessly with the rest of the loop, and apply the effects you want to each portion of the loop.