GKT - Here's another hint. Chuck the entire pot and use a center drill to start the hole in the aluminum. Remove the pot, clamp it lightly in a bench vise, and restrain the stem with a pair of needle-nosed ViseGrips, then finish drilling and tapping the hole on center. Obviously, the "new" end of the stem is easy right in the chuck.
I would hesitate from putting much force on the mechanical stop within the pot, but a light center drill marking the spot shouldn't place undue stress. We're only going in to aluminum, but we all know how that can grab the tooling.
You could use a 3-jaw internal set but would need to machine one helluva deep/thin step in the reverse side to clear the diameter of the pot housing.