should be able to create ustar archives (i.e. non-posix/non-GNU)

Currently, the tar option will always create POSIX archives or GNU archives.
However, not all implementations of tar can handle this: https://savannah.gnu.org/support/?109437

AIX 7.2 will unpack Directories named PaxHeaders.X, not sure about what happens with GNU tar archives.

However, given that ustar is more universal until AIX 7.3.1 arrives, it would be a reasonable option to support ustar as well. Even after that AIX release, it might be a reasonable choice for other Unix-like systems, as the posix standard has not been adopted widely.