PMU resets, power button good, but keyboard not working.

I work on these A1342 laptops as a hobby, and have successfully repaired somewhere between 50 and 100 over the past year and a half. I've seen a lot of different issues, and have become pretty good at identifying the problem when one isn't working. Sometimes I can fix it, other times I can't despite knowing the issue (component-level repairs, etc).

I have one now that's got me completely stumped. I'm leaning towards the logic board, which I hate to say since it works fine other than the following issue, but I'm hoping it's something else. The symptoms are as follows:

- The keyboard does not work once the computer is turned on. The PRAM won't reset and once into OSX, no keys work at all.

- The power button works fine and the top case is a known-good unit that I purchased brand new and have tested with 2 other logic boards without issues.

- PMU will reset fine (shift, control, option, power) when computer is off; charger light cycles from green to orange; this works indefinitely. No issues.

- Once computer is turned on, keyboard stops functioning but the power button continues to work fine (when in OSX, if I hit the power button it brings up the shut down dialog).

- Trackpad works fine at all times.

- No liquid damage apparent

What could this issue be caused by? The keyboard seems to stop working as soon as the laptop is powered on, but it appears to work fine when resetting the PMU. Could something maybe be causing a short that only causes the keyboard (and not the power button) to short when powered on? Any help would be much appreciated. The logic board works great other than this issue.