realistically, though, it's hard for me to see how setting the serial # at firmware image build time scales. And setting it after boot makes no real sense either -- it's not really a serial number if you're changing it at that point.
But some way to customize the binary images with a serial number seems most workable, if you're going to put the serial number in the firmware image at all (which never made sense to me either --serial #s are supposed to be indelible, and firmware images are not indelible).