You could, yeah. The problem is all the braking forces will be trying to pull the whole assembly down and its gonna flex like mad, the frame back thre is nowhere near as beefy as the forks are... If you could secure the plate arms to the frame with jubilee clips or big fat cable ties (thats what I did when I ran one) that would help a great deal.
Your other option I suppose is to mount it on top of the seatstays to eliminate that problem