Migration data for 5.0 's already up on the CDN servers actually :
http://ak.worldofwarcraft.com.edgesu...nContent_1.MPQ
http://ak.worldofwarcraft.com.edgesu...nContent_2.MPQ
You should start to download this if you open the PTR launcher.
If you're manually downloading those files they go under
Wow PTR folder -> TempMig -> 5.0Migration-data
No idea when they intend to open the PTR for MoP but the migration data's on the PTR background downloader. Has been since the 1st of the month actually.