I tried *both* Ethernet ports and never got a DHCP request when using the May 15 build. No flashing blue light, either, so the problem might actually be worse than just Ethernet, but I don't have serial console hardware to say for sure.
I also re-dd'ed the image just in case something miraculously went wrong the first time (which has never happened in all my years of doing this, but wanted to rule it out completely). I did not try re-downloading, but the sha256 checksum matches.
In between those two attempts, I tried the out-of-support Ubuntu version that Rock Pi ships, and that worked as expected.
I finally ended up downloading the November build of Armbian, which also worked as expected (both ports).
The problem with trying to do custom device tree overlays is the same as the problem with bringing up Wi-Fi: the config files are on an ext4 volume, which can't be readily accessed from my Mac.
Sure, I could hunt down an external SD card reader (I probably still have one somewhere) and a Raspberry Pi (I have more than a few somewhere, though only a couple of them aren't permanently installed inside something), booting that, SSHing to it, etc., but it's not worth the hassle when I can just downgrade to the November build. Sadly, my days of Linux kernel hacking are over, replaced long ago by having a hundred times more stuff to do than I have time for. 😁
I do have a proper serial adapter on its way from Amazon, and I'll try it next weekend to see what happens, assuming I remember and have time.