Thanks for the help, but I am not really familiar with linux at all. What does it mean to send a command to the "linux console shell"? Do I have to access it somehow before sending a command? I've soldered wires to the serial port pads according to the layout in this post:
https://www.mobileread.com/forums/showthread.php?t=49942, and I've tried sending some echo commands as you suggested but I'm not getting any response (115200kbps, 8-N-1). I've also monitored the TX line with an oscilloscope when starting it up, but I didn't see any activity.