Well folks, I was surprised to find my supercharger pulley has disintegrated. I didn't know it was non-standard, I bought the car with no known history. Apparently it was well tuned - pulls very hard and gets reasonable mileage. The pulley was a 65mm aluminum one which split in two along the root of a belt groove.
After looking at options, I bought a used stock pulley with a new bearing already installed, from NeedsWings.
The picture shows the first step of turning it down. Workholding was a challenge, this is a third attempt, which worked well. Next I flipped it around and held the nose in the 4-jaw. Dialed it in and made the rest of it bald. I cut the new grooves with two custom ground tools. I only took it down to 67mm. If my measurements and calculations are right, there is a little more than 3mm of wall thickness remaining at the root of each groove. A 65mm pulley would have only a bit more than 2mm, which seems risky to me. Hopefully 67mm will work okay with the tune.
This is by far my most ambitious machining project. I was worried that the stock material would be a stainless that work-hardens when cut, but happily it is pretty mild and very free-machining. I'll install in the morning and add a picture of the result.