Trigger and arguments question

Hi everybody!

I have a trigger like this:

CREATE TRIGGER mytrigger
   AFTER INSERT OR UPDATE OR DELETE
   ON myschema.mytable
   FOR EACH ROW
   EXECUTE PROCEDURE myschema.myfunction(myarg);

It sends an argument to myfunction(), and I can retrieve this value in
TG_ARGV[0]. Fine.
What I'm trying to do is using TG_ARGV[0] to point to a field in NEW or OLD.
Is it possible?

Something like NEW.TG_ARGV[0]...

I'm trying to write a kind of generic function which I could use on multiple
tables with different field names (myarg being the field name).
But I can't get it to work.

Any clues or other solutions?
Thanks,
-- Hervé Inisan.