I used a custom u-boot because my K4 had lifted pads for serial input. It was "output only". So I used a custom u-boot with hard-wired idme vars. But with fully functional serial i/o, the idme var commands would certainly be easier than building a custom u-boot.
But you still need to know what values to put there. The amazon techs should be able to tell you your serial and pcbsn, if you can get escalated to a high-level tech who has access to that information.