You can actually fit one of those thermalright ultra-120 extremes sideways in an x-qpack, if you cut off the little spar that supports half of the hdd cage. As such, it can fit most every other high performance cooler as well. I've seen folks fit zalman cnps9700s in their x-qpacks as well. So yeah, you can run a quad in one fairly easily.
Do you have any links, pictures, or info on how to mod the qpack to fit a 120 extreme? I'm wondering also if you lose some HDD slots by modifying.