Update: Found this youtube video: youtu.be/xJENJNz_JLQ
which gave me the BMW part number for the transmission. Then I found this site (link)
which gave me a labeled diagram of the trans, including the full specs on the plug thread: M18x1.5x12
. Searching around, it turns out BMW uses those plugs quite a bit on their engines and transmissions. I suspect it's a European or metric standard plug. I have a suitable one on the way from another vendor.
This was all done from the comfort of my desk so I will have to confirm the fit when my parts arrive. Just wanted to give an update and share the information I found.
Queue the music!