This would depend entirely on the standards of the community. Most online communities keep a pretty low tolerance of racism even in satire, but there isn't a right or wrong answer for a community other than whatever community guidelines that community chooses to set.
It is important to remember that tone doesn't translate well through the Internet and some people aren't good at recognizing sarcasm and satire, particularly since satire often tries to ride the line between serious and sarcastic very closely.
My recommendation would be to avoid it, certainly outside of any context that isn't blindingly obviously satirical, but it is probably easiest just to avoid it altogether that way you don't end up with a "how far isn't obvious enough" question that is hard to deal with.
Also, keep in mind that even though using racist language and racist scenarios to fight against racism isn't racist in and of itself (as the intent is to promote equality), it is still generally offensive. Unless it is being offensive to try to make a difference, it is probably ill advised. Similarly, it's probably best to use the least offensive subject that can still get the point across. (For example, since white's don't have the same kind of history of prejudice in the western world, they make a good target to be used in satire of racist situations, but that might not work for all satires.)