This is insightful especially since we have thought about this problem and had not come up with a solution. I am assuming all the icons will be unique though, else it may lead to a lot of confusion. Also, I believe people choose icons that they best identify with and if it is not unique their association with the icon may not be as strong.

Another thing I have experimented with is if you can upload a picture and create a hand drawn painting / image like Chris Dixon's or Fred Wilson's image which I think are brilliant in creating a brand and identity.