My first choice would be an USART, using a spare one through test-pads. You have a number of options for transferring this information to the micro-controller, apart from the obvious interface where you actually intend to use MAC address in the first place. This would also be a good time to introduce the unique MAC address to the system. Presumably this process would be conducted, at least in part, by specialized software you have loaded onto the micro-controller, either specifically for this task, or as a 'service mode' in your release software. If you are going to be making more then a handful of your products, it is reasonable to assume there is going to be some kind of Quality Assurance testing and perhaps calibration.
