Those have been done for many years, bumpin-thumper.
I have a 2006 crew cab, and it's just about impossible (not to mention impractical) to try and put a box under the rear seat. However, by removing the jack, tire iron, and those vents, you can fit around 2.5 cubes behind the seat with a max mounting depth of 4.5".
I've got some RE 8"s in mine now, but am waiting on the new Stereo Integrity Mag's to come out. They're 12"s with only 3.5" of mounting depth but with amazing SQ and output to boot. They should be out within the next month, and best of all carry a price tag of only $149 each.
There are no other shallow subs out there that can compare with these subs, especially when you factor in price.