I think I figured it out.
I was doing this
select sp_session_id_seq.nextval;
instead of
select sp_session_id_sequence.nextval;
It is not clear that you need to change the db_nextval code to refer to the next sequence name.
Since this is called from a TCL page and is not pulled into a query file, should be change the Oracle data-model to use the same sequence name? That is sp_session_id_sequence instead of sp_session_id_seq.
