If you're supplying +5V DC on pin 9 of the cable, you'll have significant voltage drop over that length & it probably won't work.

An external keyboard or mouse port has +5 and ground on it. Here's a pinout:
http://www.indiacam.net/pinout/pinout_ps2_serial_mouse.htm#3

I hacked a serial cable and old mouse together to pull +5V from the mouse port and provide power on pin 9 of the serial cable.

You could probably do the same ...

I have an LK202-25. When I short the jumper near the DB9, the unit powers up, but the backlight still doesn't work. I can run a wire from there and jumper over the diode between the power connector & the internal power bus, then it works. Is this safe to do?

BTW, there's no mention of the jumper in ...