Thanks for the further information. As you can see I wasn't kidding about lack of familiarity with BPi.
It seems to me then there are two issues. The first is getting u-boot to attempt to boot off the HD. This I know nothing about. Second, though, if you can resolve the first issue, is that I think you'll still need to have the primary bootloader installed on the hard disk. It is placed at the start of the NetBSD partition. Without it present, I doubt you can boot NetBSD.
Hopefully you'll get some replies from others more familiar with this particular setup.