What's the best way to transfer ALAC files between computers when subscribed to iTunes Match?
Turn off iTunes Match on the PowerMac.
Use Home Sharing to move the ALAC files to the PowerMac.
Re-enable iTunes Match on the PowerMac.  It should re-scan the library and match the ALAC songs.  
Don't ever delete the ALAC songs from the PowerMac, otherwise your only choice will be to download a 256Kbps file from the cloud.