Extending postgres objects with attributes

Several times I wanted to "extend" some of the postgres objects, like roles 
or functions. For example, sometimes you want to add extra attributes to 
roles, which are application dependent. Or sometimes you want to store 
functions and reference them in your custom tables, without losing 
referential integrity.

Now, postgres doesn't allow it's objects (from system tables) be referred to 
in your custom tables, thereby making the process of extension difficult. 
(cf. http://archives.postgresql.org/pgsql-general/2004-12/msg00840.php) So I 
wanted to ask here how people solve/implement such "extension" requirements?

Regards,
Davor

PS using inheritance in this scenario is problematic. U can for example do 
this: CREATE TABLE tbl_privileges ( priv_execute integer ) INHERITS 
(pg_authid); but the system internally will probably always use FROM ONLY 
pg_authid, so it will not see the users from tbl_priviledges. IN other 
words, those user will not be "real roles". Note also the fact that in this 
way, pg_authid could seem to have it's UNIQUE constraint on rolname 
invalidated (besides the other INHERITANCE caveats...) And finally, I wonder 
if inheriting a system table is even safe...