Thanks for all the input, guys - managed to get it done this evening.
First I rigged up a blow gun to an -6AN barb fitting, along with a zip tie, to continuously blow air out of that chamber while drilling:
After stepping up to 1/2" in 1/16" increments, tapping the bore to M14-1.50, and replacing the provided copper washer with an appropriately-sized nitrile O-ring, we have this:
Unfortunately my dumb ass got hasty when tapping the bore, and realized after a few threads were cut that the tap was cockeyed. Backed it out, applied more pressure, and carefully ran it back in... Given the extremely limited threads on the VDO 323-121, I'm concerned that it may leak at some point (and it's internally-grounded, so using thread sealant could yield problems).
Oh well. If it does leak, and I can't find an effective way to seal it, I'll just remove it and plug/epoxy the damn hole shut (or find someone to TIG a plug in). Really pissed I didn't exercise more caution, but there's a chance it'll be just fine.