(trigger function) -> ERROR: NEW used in non-rule query

I'm writing a trigger which wants to reference the pre-defined NEW record.

I can do this
idval := NEW.blah;

This works fine!!!

What I really need to do is
idval := NEW.(quote_ident(TG_ARGV[3]));
or this
idval := NEW.(TG_ARGV[3]);

unfortunately this returns
"ERROR: NEW used in non-rule query"

I've also tried using
EXECUTE ''SELECT NEW.'' || quote_ident(TG_ARGV[3]);
(to test if I can use a FOR-IN-EXECUTE to extract it)

and it produces exactly the same error. Obviously NEW goes out of scope 
for some reason. I can understand the case with EXECUTE but the other 
statements should have worked :(

This is very frustrating because it is the *ONLY* thing standing in the 
way of me solving a larger problem (which I will use to repsond to 
another thread in this forum).

Does anyone know how to access an arbitarily specified field of the NEW 
record in a trigger function. I need this because arguments to a 
function in the trigger definition cannot be NEW.blah, they have to be 
static/literals (which is likely to be for the same reason that NEW goes 
out of scope any time I try to do something dynamic when evaluating a 
field on it).

FYI: ref doco at
http://www.postgresql.org/docs/7.3/interactive/plpgsql-trigger.html

using pg 7.3.3

-- 
Terence Kearns ~ ph: +61 2 6201 5516
IT Database/Applications Developer
Enterprise Information Systems
Client Services Division
University of Canberra
www.canberra.edu.au