audit trail and system catalogs

Hi,

we strongly need to implement an audit trail as a prerequisite for 
clinical trials, that is a functionality which records any change of any 
data item in a database into one single table containing these events.

I found that one could use rules for this, but this would require one rule 
for each attribute of each table ("...ON UPDATE ... WHERE old.attr <> 
new.attr...").

My first question:
Could one reduce the rule set with rules for tables, e. g. one rule which 
fires whenever *some* attribute of a table changes? Would there be a 
possibility to reference the changed attributes in the action clause?

Assuming that one really needs one rule for each attribute I would like to 
create a script which generates all rules for all attributes of all tables 
for a database.

The problem is, how to get the names of all tables and their attributes? I 
looked into the system tables ("pg_*"), but there were many tables and 
many attributes for *my* tables, and I did not figure out how to 
distinguish my tables and my attributes from the other ones. None of the 
columns in pg_class and pg_attribute seems to give information on wether 
the item is system or user defined.

My second question:
How can I loop to all of *my* tables and *my* attributes, ignoring system 
tables and system generated attributes within my tables?

Thank you very much for any hint,

Markus