Thanks for the replies. The link to the Rockchip boot sequence is definitely helpful.
On running the split boot, that is kind of what I was trying. I have an SD card (with the official Armbian distro for the R2S) in the slot, and it starts the boot from that. I then interrupt it at the u-boot prompt, and then do "usb start". It loads the driver, sees the onboard USB hub, but never finds any device attached to the USB port. I know the NanoPi R2S is a recent board, so maybe there is some missing or not supported?
I also tried this with the official distro from FriendlyARM, but that version of u-boot is really old (2014 according to the version number) and does not seem to really support USB.
I would be happy to test things if anyone has a suggestion. Thanks for any help.