As long as there is no PMS version for older G5/G4 PPC Apple machines you can try "MKV2VOB" on a Windows PC:http://www.mkv2vob.com/showthread.php?tid=1
Without transcoding it changes the *.mkv container to *.mpeg
You can then stream this files even from an older Apple perfectly to the PS 3 using PMS
A HD movie takes about 10 minutes to be processed on an quite old 2.8 GHz Pentium 4.
At least one reason to still have a PC around