My two.......
I dont use an MPC, but I do have a sampler! I've done a few guitar sampled cuts.
Depending on how complex the guitar riff is, I usually lay the drum track down on my recorder, then have the guitarist play over it for about a minute or so. If its complex, I just leave the riffs on the recorder and with with on there! But as far as sampling, once we get a few solid takes, I go threw each separate riff like the chorus part, verse, intro, etc., and find the best ones, from there I sample them to my sampler (well actually I import the wav file to it! Dont know if you can do that on an MPC!!) For me it comes out flawless!
Some advice no matter what!, if you can, get two separte guitar riffs for each part, like two separtly recorded loops that will be used in the verse and channel them left and right, it really makes a dope stereo sound! Good luck!
PM