pax(1) does not support the pax interchange format. Support is both useful (e.g., to get around the 8GB file size limit of the ustar interchange format) and required for conformance with SUSv3/SUSv4.
Ideally, use libarchive(3); although it would probably be more straightforward to implement the (partial) pax interchange support from Mac OS X pax:
How-To-Repeat: $ pax -x pax
pax: Unknown -x format: pax
pax: Known -x formats are: bcpio cpio sv4cpio sv4crc tar ustar
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Status: In Progress Changed: (is less than) 2014-06-01
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