Charlieman and I recently discussed this on another thread and to our knowledge we have not seen a specific reference to it. It is entirely possible that Apple didn't use Pantone to choose "Platinum", that the Macintosh beige was a fluke, i.e. the plastics manufacturer gave Manock some PMS swatches when he was looking for a new colour, which is why we know it – meaning not all manufacturers may have used the Pantone system, at least exclusively. Knowing Apple's NIH (not invented here) philosophy, they may well have created their own copyrightable color at that point (FYI, Pantone colors are officially copyrighted).
I am collecting well preserved samples (not yellowed) of all of the original Apple systems with the intent that I will go to a colour matching service and get specific identification of the colour variances. As far as painting a Platinum case, I would take the same approach. Most types of paints, including vinyl dyes, should be able to be custom tinted at your local paint store. It's worth a try to take a pristine Platinum case part (like the inside of an external drive case) to your local paint store and see how close they come. I have actually been shocked by how close my Home Depot came to matching period paints used in my house during restoration. Even with a different gloss base, the new dried paint was indistinguishable from the original.