Very glad it helped! The idea came from a correspondent on the freebsd-arm mailing list, not my own insights...8-(
There still remains need for a readable u-boot manual. I have figured out that even if a Pi (3 in this case) has no microSD card and still managed to find and start u-boot on a USB device it's unable to save environment variable to the USB device, it still wants to write the change to microSD.
Perhaps recompiling u-boot with appropriate options will either make the location of uboot.env selectable, or maybe there's a more clever trick. If anybody knows please make a noise.....
bob prohaska