Really it has do do with the MB remotes being primarily IR. The newer keys use both IR and radio, so that improves the range a little bit. But if you have the IR only key, the only way I can think of to increase range would be one of those IR extender devices used in home theater...but that might be a little bit unwieldy.
But hey, what an idea...just mount an omnidirectional mini sensor to your headliner/roof, run the cable to the repeater, find a way to feed it power without draining the battery (separate battery? or low enough voltage?) and have the repeater hooked up to either the rear view or door sensors. hmmm, now who would have the balls to pull that one off?
A better way to go might be to just get an aftermarket alarm system to work with your car...but that might mess with you being about to use your MB key.
http://www.nilesaudio.com/products/infrared.html