The oppression within the cosplay community is just a reflection of "real world" oppression. "You can't cosplay if you're black", "you can't cosplay if you're disabled", "you can't cosplay as a different gender." These statements are the result of an existing oppression/discrimination in society. Just replace "cosplay" with "get a job easily" and you have real life. Not at the fault of the PoC, disabled people, or trans* people, but the fault of the employers' discrimination. It still happens and it's sad. It's hard to fix it within the cosplay community without fixing it within society in general. But just encouraging the idea that anyone can cosplay helps. Don't let people get away with saying things like "A black version of (character)", or "(character) in a wheelchair". Don't limit cosplayers to their race, health, physical/mental condition, or gender.
In fact, we have a perfect example of the result of this right here: http://www.cosplay.com/showthread.php?t=352117