Actually ABX has been doing this from Day 1.
All their DC9 and DC8 were also loaded through standard pax doors.
The original idea used the trolleys that they used in hospitals for collecting the dirty linen.
It is a tube frames with wheels and a bag slung between the two sides of the frame. It was just wide enough to go through the pax door. <img src="wink.gif" border="0"> <img src="wink.gif" border="0">