i do this, basically use my pc as a sound module.. so long as you have a midi input on the computer you can then have at least 16 simultaneous midi tracks playing at once.
however! it'll be really difficult to use cubase as i'm not sure you can use more than 1 vst at a time (assuming you will be sequencing on the mpc anyway, i don't think cubase can assign specific midi channels to different vst's) - i use a program called v-stack which is made by steinberg, it's basically cubase minus the sequencer - you can still load vst's, plugins etc but it's designed to be really easy to split incoming midi into it's respective channels.. it's dope. i've never used pro tools before so i dunno about that..
of course, you'll still need a way to mix your computer output with your mpc output, i use a little 2 channel dj mixer going into a spare input on my soundcard, but you'll have to figure a way to do it on your set up..
here's some more information about v-stack if you were interested anyway:
http://www.steinberg.de/ProductPage_sbf ... ct_ID=2130