Don't call it formal rush, that will scare the guys to death!
Try calling it "structured rush" or "open house rounds." What I think would be good to try is to begin rush with an open house night where every rushee has to go to every house. After that, they can go where they want. The key is for the rushees to see every fraternity, not just "oh, Joe Bob the XYZ is in my class, I'll go to XYZ." Plus there are some guys who are shyer, and might be a little scared to just walk into a party. If they had the structure to start out with, more people might be encouraged to get involved.
If people are worried about their house being crappy or whatever, propose having this in the rec center or other neutral territory. The point is that guys will see fraternities they probably would have never taken a look at otherwise. That can only be beneficial for everyone.
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