I tried attaching a 16x2 LCD to the UPS control panel but no dice.
The backlight does turn on and off as expected. On when the unit is on, off after a timeout, and back on when a button is pressed. Otherwise, it's a blank screen.
The LCD does work when paired with an adapter, so that's not the issue.
The UPS control panel is driven by an STM8 and is fed data via SPI from the main UPS unit.
A few potential causes:
1. The backlight voltage from the UPS panel is 12V, which is much higher than the expected 5V. There are current limit resistors, but the backlight pin voltage (15, 16) does drop to 6V. Not sure if this is affecting the control panel itself.
2. The supply voltage from the UPS panel is 3.3V, whereas the display may be spec'd at 5V.
3. The LCD contrast pin (pin 3) may need tweaking, but there's no pot on the control panel board.