I can see where your coming from here Luke. I think the shorter focal length will be more a 'who has the most nerve' competition rather than a street portrait competition. Personally I prefer to take portraits at a long focal length to compress the perspective and get a nice OOF background. While this is possible with a shorter length, I feel it would take away my own personal style of portrait photography whether that be in a studio, or on the street trying to get someone while there are going about their own business.
I feel your idea need slight tweaking to appeal to the masses as there does appear to be some negative emotion towards your suggest idea. Plus with the differing crop factor as mentioned by other esteemed member of this forum, your recommendation of 85mm maximum vary between camera bodies as to how far a participant could be from their desired subject of the competition.
I feel you would be wise to lift your restriction on this occasion, to see how popular your competition could be. By all means you continue with your 50mm, they are great pieces of glass. From reading here, potential entrants would be happy and more likely to participate, if they could use there favorite piece of glass for your challenge.
