I stopped by a service center today, and innocently wandered into the repair area. I asked one of the guys about the HEPA filter, for all us non-PUP folks, and he said it is one of their most constrained parts, and that they would likely offer it for install once the part was more readily available.
This appears to be confirmation that Tesla isn't artificially preventing us from ordering the filter to boost PUP sales. He said to expect it to be available for individual purchase in the near future, pending availability. (First post to the forums, FYI!)