After dinking around, I stumbled across a copy of a boot.ini file online that had far more video mode options.
Found one that matched my 1366x768 resolution precisely, so I copied the lines into my XU4's boot.ini file.
1366x768 without monitor data using generic information
setenv videoconfig "drm_kms_helper.edid_firmware=edid/1366x768.bin"
I then made sure all other video configurations were once again commented out.
Guess what? Didn't work. My TV is still just as dark as the first day I tried this out.
I'm going to re-image again. I know this specific flavor of RecalBox for the XU4 is a fork and y'all probably aren't the best folks to reach out to when I'm running your OS off of super-specific hardware, but jesus it's hard to find sensible documentation.