How to implement an auto-increment column for a system catalog table?

Hi all,
I am using Postgres to build the prototype in a research project. I need
to create a new system catalog table with an auto-increment column. For
a ordinary table, CREATE SEQUENCE or a serial type can be used to
implement the auto-increment column, but it seems Postgres do not
support creating a sequence or serial types in system catalogs. How to
implement an auto-increment column for a catalog table?

Thanks in advance.

Wang