Just curious as to what your solution was as I have the same scenario and am presently in the process of setting up one master with multiple replicas housing home directories and also mobile accounts.
I am also curious as to how your final setup works. I need to set up this exact scenario: Two locations wtih a master and replica OD server. The home directories will be localized to the server that is local to the user, but when a user from site 'a' visits site 'b' they will need to be able to login to their network account to work.
All comments and advice are welcome.