Referencing columns from system tables possible?

Hello pgsql-general,

When trying to create a table

CREATE TABLE sessions (
 id serial PRIMARY KEY,
 procpid int4 REFERENCES pg_listener(listenerpid) ON DELETE CASCADE
 );

I get a warning saying 'relation "pg_listener" is a system catalog'

Is it unreasonable and/or impossible to do this?

--
-Boris