While it's great that people have all these ideas I think you need to be careful about imposing too many rules and regulations - as soon as people have to start jumping through x number of hoops they won't bother and it'll just drive them away.

Perhaps the simplest (or least intrusive), way to retain the 'essence' of Bryce in a render would be to do away with postwork (much like this months challenge). At least that way you're using Bryce to set the tone, lighting and ambience of the scene rather than relying on image editing software to achieve it.

Maybe even shift the focus of the challenges from learning Bryce to just having fun with it - if you're having fun the learning side of it will just happen anyway and you're more likely to participate in the challenge. Everybody wins!