QwertyChouskie

Turns out /dev/ttyS1 works perfectly!  I was thrown off by the fact that it exists whether the overlay is enabled or not, but only works with the overlay enabled.  With the overlay enabled, using /dev/ttyS1 worked perfectly.

Have you tried the generic rk3328 builds found discussed in this thread: 
 
 
I would start there by reading that thread.  Your box likely has a lot in common with what is discussed there.

Good news!

Hi,
 
if it is similar to the tinkerboard, you should enable the overlay using "overlays=uart1", and the /dev/ttyS1 should be the serial port.
What does "dmesg | grep ttyS" say?
 

Turns out /dev/ttyS1 works perfectly!  I was thrown off by the fact that it exists whether the overlay is enabled or not, but only works with the overlay enabled.  With the overlay enabled, using /dev/ttyS1 worked perfectly.