Enforce primary key on every table during dev?

We want to enforce a policy, partly just to protect those who might forget,
for every table in a particular schema to have a primary key. This can't
be done with event triggers as far as I can see, because it is quite
legitimate to do:

BEGIN;
CREATE TABLE foo (id int);
ALTER TABLE foo ADD PRIMARY KEY (id);
COMMIT;

It would be nice to have some kind of "deferrable event trigger" or some
way to enforce that no transaction commits which added a table without a
primary key.

Any ideas?

Thanks,
Jeremy