I was just referring to the RK3399 boards (given the extra regulators for the A72s and GPU, it's apparent that the RK808/818 was designed for a less power-hungry SOC), but yeah, it's pretty obvious to me now (!) that the implementations vary. A lot, considering the relatively minor variances, mostly in the peripherals (audio codec, W-Fi, eMMC, etc.). The Realtek Ethernet phy is pretty much a standard, at least. What a tangle. Nothing that can't be solved with enough time and money. It's too bad both are tough to come by.
The Orange Pi RK3399 has 5V and 3.3V available on the 40-pin header (and nowhere else that I can see offhand). Pinout is in the manual; it's a standard .1"/2.54mm header - lots of connector options. The mSATA interface is a bit inconvenient (uncommon); it also has 3.3V power available - perfect for an mSATA device; not much use if you're adapting it to a 2.5" device. I didn't check to see how much power is available, but I'd expect it to be fine for typical devices.