this is quite a sought-after mod but i'll tell you that unfortunately, it's not a realistic option for the monsters. it wouldn't be terribly expensive to get some custom tubing bent up to run modified 748 cans under the seat, but the main issue is that there's not nearly enough rear-wheel clearance to have mufflers between the frame and rear wheel. one good bump, or one day with a passenger, and you'll be rubbing the wheel against the pipes HARD.
if you REALLY want to go with this, you could modify your frame. or just cut the rear of the frame off and graft the subframe off a 748/996 to your monster to have the added clearance. but then you get into custom seat fitment, and all sorts of other obstacles.
if you haven't done so, take a look at some high-mount exhaust options. there's a bunch of cans on the market that run up high along the seat. not UNDER the seat, but they look pretty sharp