Yes, absolutely no information anywhere on the internets about a “HSP” format.
Really though, it’s more about the Hybrid Log-Gamma TRC. This is just a guess, but it’s probably something like, Panasonic wanted to produce still images with this TRC, but couldn’t encode that fact in the metadata, so they took, say, PNG, applied HLG to the image, and saved it with a .hsp filename so their devices know to deal with it as such. Video streams, the original target of HLG, apparently can accommodate such without shenanigns.
I just pushed a rawproc change that adds a ARIB STD-B67 HLG (0.0-1.0, not 0-12) curve to the tone tool, for anyone who’s cloned the repo. Straightforward interpretation from the Wikipedia definition; nice in that you don’t have to do stupid pet tricks to make log behave approaching 0…