We have the Odroid in hand and it's working great. I was able to get my whole software stack running on it, which is really great.
In the mean time I've been conversing with the Radxa folks on the rock2 square and was able to get the GPIO sort of working. Currently it's using the sysfs system in Linux, which is pretty standard. The issue now is that I don't have a mapping for GPIO numbers to actual pins. If anybody has this information, please let me know where to find it as it would be nice to get the rock2 GPIO working. The custom top plate that we machined fit's it perfectly.
For now we've moved to the Odroid and installed it in the Singularity Robot. We are currently blocking on a 40 pin cable to connect to the board. We are using the Odroid with the level shifter board on top mounted underneath the robot top plate with the antenna coming out the top. This means there isn't room to simply plug things into the header pins. We must use a ribbon cable, which is arguably better anyway. The nice thing about the Odroid is that the level shifter is labeled with GPIO numbers. This makes it simple to know what is what, as the numbers are pretty sparse and all over the place.
More news soon.