If I am on another OS and for some reason I need to take a quick look inside an HPKG, is it compressed with a standard format (zip, tgz, rar etc) or is it proprietary? Sometimes I get a query by email and it is a schlep to fire up my Haiku box when I might be able to just open up the HPKG and see what the problem is.
It is a proprietary format. The reason for this is it is designed to allow random access to the archive contents. So, it is compressed by 4K blocks, with an index at the start to allow easily reaching each block.
However, it is possible to compile the “package” tool for one of our supported host systems (Linux, BSD, Mac OS X) and then use it to extract the packages. IIRC you use the --host-only command line flag to Haiku’s configure script. There is a tool for windows floating around, too.