default_variants per platform

Is it possible to set "default_variants" per platform ? Basically, I want to set +aqua for platform macosx, and +x11 for platform puredarwin or platform freebsd. I tried the most straight-forward approach, wrapping default_variants in platform, but it didn't work... For now I made due with "variant aqua requires macosx". I also seemed to have the misguided notion that the default variants would somehow go away when actually providing some variants ? Not so, I had to give an explicit "+x11 -aqua" to get rid of the defaults... --anders