The tl;dr version is: IT WORKS!
I had this wild idea some months ago when the Pine64 board was just another project on Kickstarter to be able to set the board in its enclosure (which I haven't received yet, there was a delay), and let it control an Atari. I'd either use SIO2BT or SIO2PC over Serial or USB, or some combination of the two. My first trial was using the Pine64 board (the 2GB Pine64+ model), running Ubuntu Mate 16.04, with the Sabrent SBT-FTDI model adapter and connected into an SIO2PC serial jack. AND IT WORKS WITH NO CHANGES to the current Github copy of the RespeQt client.
I had to install Qt5, g++, and set up a symlink for qmake, and that was it. RespeQt launched...but then I had to get it to talk over /dev/ttyUSB0 and restart it with sudo. Screenshots attached from Ubuntu, and a photo of the PIne64 board connected to my SIO2SD + SIO2PC Serial box. Thanks everyone for the continued work!
I'll be trying a raw Android build and/or Remix OS and SIO2BT next.
Edited by JohnBuell, Mon Jul 4, 2016 8:10 PM.