Jason,
There is already an ehancement request to track this issue :
https://glassfish.dev.java.net/issues/show_bug.cgi?id=2021 . The initial native sequencing support added to the PostgreSQL platform was for the SERIAL data type which is effectively Identity type sequencing and SEQUENCE type sequencing is not supported. You could update PostgreSQL platform to support a SEQUENCE (compare it to the Oracle platforms) and provide your platform within the persistence.xml properties which would provide you with a much more simple workaround.
You could also switch to a SERIAL datatype for your PKs or Table based sequencing.
--Gordon
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