I think such filtering is done via DBMS_DATAPUMP.metadata_filter.
Various filters are available including one called EXCLUDE_PATH_EXPR.
There are object paths that can be viewed by selecting from
DATABASE_EXPORT_OBJECTS, SCHEMA_EXPORT_OBJECTS and
TABLE_EXPORT_OBJECTS.
However I don't know which of these correspond to those that would be
excluded if using EXCLUDE=grant with impdp.
I also have not been able to find this out on the web.
Does anyone know exactly what impdp's EXCLUDE=grants does in terms of
the DBMS_DATAPUMP api?
Thanks.