I am no engineer but I would guess the mounts for the struts are as strong or stronger than mounting something to the strut. Why not just have someone weld in a bar? It won't be removeable but should do the same thing (which is about nothing, the whole back seat area is a big strut bar).
Also look into and "ikea brace". It is basically a big *** peice of sheet metal that fills in the openings behind the seats. It is the same idea as cheap ikea furnature that needs a board on the back to keep things from collapsing. Another cheapish alternative.