Interesting post Dwill!
I would have done only one thing differently the rsync from old to new host I would have done live i.e Old server still running hot sync...
Once all is across the net then another hot sync, then stop zimbra on the old server and then a last cold sync. After that old and new server /opt/zimbra dirs should be in sync, and now run the zimbra installer to upgrade the new server install.
But I like your approach!
heinzg