And of course as soon I posted this, I found my issue. The keyboard 5v line was the culprit. I'm about to boot it up and test, but for sake of satisfying potential future Google searches, here's my fix:

Well, I'm giving up for tonight. The system will now boot with the keyboard attached... sometimes, but it's throwing a keyboard clock line error. I'm assuming something is wrong with my connections, which I'll have to test further.

Also recorded a video of the machine not wanting to boot with the keyboard attached: https://youtu.be/s83sDTyubFw

but the original trace thats burned is not connected, I guess can't tell without looking at the other side, but why not connect the jump wire to exact where the original trace went?

treeman wrote:

but the original trace thats burned is not connected, I guess can't tell without looking at the other side, but why not connect the jump wire to exact where the original trace went?

The pad on the other side of the via is barely attached to the substrate and I didn't want to accidentally tear it off. Continuity shows that it's the 5v line, so it shouldn't be a problem.

Sorry for the bump, just wanted to update with an answer for anyone that has a similar problem.

As I stated previously, I plugged a KVM in using a PS/2 to AT keyboard adapter and it completely burnt up a trace. This trace was for the 5v line. The pin I connected to in the second picture was not the 5v line. I'll have to take a photo later to show the actual repair. Unfortunately, it didn't just burn that trace up.

The keyboard controller also was damaged. I determined this by noticing it was throwing keyboard data errors instead of just "keyboard failure" after fixing the trace. This motherboard uses a 44 pin PLCC-packaged N8242PC keyboard controller which I was able to find on eBay for ~$5.50 shipped.

I had to desolder the chip and put the new one in, but after that it works fine and I now have a working 4DX2-66V!