DBMS_DEFER_QUERY.GET_datatype_ARG NULLS

I'm trying to distinguish between NULLs returned from calls to DBMS_DEFER_QUERY.GET_datatype_ARG that are NULL because I have chosen to use min_communication = true (when calling GENERATE_REPLICATION_SUPPORT) and those that are from setting a column to NULL. I can't see any way using the API provided to get at this information, but it must be known internally to apply the transaction.

Is there any way to distinguish between a column being set to NULL and the column being unchanged at the originating site?

I'm using Oracle 9.2 currently.