I honestly don't know why some people bother filling knot holes. As long as you have left enough compensating wood around the knot, you have the same amount of wood doing the work in tension, and compression, as a bow with no knot.
I'd guess a plug might work(at least in compression), if you haven't left enough wood around the knot. In this case the plug wouldn't be anywhere near as good in tension strength at that point, and could lead to a break.