Jen Grier wrote (in fact, cross-oposted to way too many groups...):

I'm writing a piece that would involve playing back what a performed had
played just a few seconds ago at a time that I must specify/que on-the-go.
The played-back segment would have effects placed on it, possibly played
back backwards and other such machinations as I see fit. Does anyone have
experience with this, or any suggestions? (My apologies for the
cross-posting, as I'm not sure where this topic is best suited)

Look into Ableton Live.

--
ha