Well, that was a real "duh" moment....shoulda thought of compressing it myself . Thanks, that gave me some breathing room.
We've got all 4 DTivos upgraded to 6.4, and MovieLoader up and running on the always-up linux machine. I have the ty-enabled ffmpeg available on this machine, and grabbed the ty+2tmf.pl script from around here. All I have to do is script it up in some fashion, and it looks like we'll have MRV and tivoserver all replaced as well.
Many thanks to crashHD, PlainBill, and all who struggled with this before us.