It turns out there is no well established metadata standard for PNG's...

Yes, isn't that strange? Has puzzled me too (even if ExifTool, I think, has set some sort of standard?). But then, (standardised) meta-data in general, is not very often a priority. About the GEO data, I certainly see the pricacy issues (could be a good idea to remove compass data when posting .sh3d files). ok