No, it won't. He wants to be able to run box A with the MAC from box B. That means that any and every time he wishes to do this he has to:
- switch off B and ensure it stays off (no waking up for any recording)
- start up A
- login to A and edit the MAC-setting file
- reboot A
- use A
- when finished using A, login to A and edit the MAC-setting file back to its own seeing
- switch off or reboot A