Maxpower3 Thank you so much! I can confirm this works on my Thinkpad P1 gen1 20MD (LPM-1 MB 17870-1, MEC1653-BA1). By extension, this should work for the Thinkpad X1 extreme gen1 as well.

This motherboard has two LPC debug ports, LPC1 and LPC2. My first hour of trying was fruitless, connecting ground to the correct pad on LPC2 didn't do anything at all. Two days later I took a better look at the the board and its schematic. Following the trace I noticed the resistors connecting the LPC1&2 pads to LAD0:3 aren't actually populated. I've been connecting ground to nothing! Upon noticing...