The problem with these drain plags are that the contact surface (which is also the diameter of the drain plug at its max) is considerably larger than the area where you turn it (the hex where you put your wrench). A mechanical engineer could probably explain it better, but you probably get the idea.
This used to happen a lot on the fill plug of Honda car manual transmissions when I used to work on them. Try this: get a 6" chisel (or bigger, if that's what you have), and bang a notch into the outer rim of the plug. Basically, you'll point the chisel directly at the center of the plug, and give it a few whacks on the edge to put a notch in it. Be careful that you aren't hitting the oil pan (sic). Once you have a notch, angle the chisel in the notch so its pointing in the direction of "loosen", and whack it again.
An enterprising owner might use a dremel to cut the notch, and then use an air hammer to spin the plug. Just don't use the air hammer on full blast!
71hondacbtwin