I'm not familiar with the Prius rear wiper, but perhaps just changing out the stock rear wiper blade with a more aerodynamic blade would suit your needs (and be safer, and legal).
http://www.valeowipersusa.com/index.asp?pgid=19
I run Valeo Ultimate wiper blades on my Ford Escape Hybrid. I didn't change to them for aerodynamics ('cuz I'm driving an Escape Hybrid, afterall... not a sleek-profiled Insight or Prius), but they do boast being better aerodynamically than most...and they sure look it, and grab the windshield even at hwy. speeds better than my stock blades did.