There is an attempt to port MyPaint for MacOSX in this forum. So far binary has been succesfully made, with some (or yet to be found) problems.
Today MyPaint team is looking for a Mac developer and/or packager. If you are interested in helping them, send an email to their mailing list use IRC (#mypaint @ irc.freenode.net).
It will be a cool port! :)