I don't really know much about swap on linux or Btrfs but I ran across something that might be useful to you. It appears that you can use a loop mounted device as a swap file. It mentions that this option has poor performance but since i haven't tried it I can say if it is cripplingly poor. Anyway here is a link http://comments.gmane.org/gmane.comp.file-systems.btrfs/29437