Reference new.* or old.* in dynamic statement?

I'm trying to write a general-purpose trigger that will disallow updates
on certain fields (I could probably do this in other ways, but I have a
stubborn streak ...).

Given a table, I want to define a trigger on that table that will
"write-protect" one column by name:

CREATE TRIGGER tbl_nomod_create
BEFORE INSERT OR UPDATE ON tbl
FOR EACH ROW EXECUTE PROCEDURE
no_modification_allowed('create_date');

I.e.,

UPDATE tbl SET fld_1 = 'foo;

would be OK but

UPDATE tbl SET create_date = now();

would result in an exception.

My trigger function below attempts to create a dynamic SQL statement
that tests "old.<column-name>" against "new.<column-name>".

CREATE OR REPLACE FUNCTION no_modification_allowed()
RETURNS TRIGGER
LANGUAGE 'plpgsql'
AS '
DECLARE
tmp_stmt TEXT;
result RECORD;
BEGIN
IF TG_ARGV[0] IS NULL
THEN
RETURN new;
ELSE
tmp_stmt := ''SELECT 1 AS is_null FROM (SELECT 1) AS dual WHERE '';
FOR result IN EXECUTE (tmp_stmt
|| ''old.'' || quote_ident(TG_ARGV[0])
|| '' IS NULL'') LOOP
RETURN new;
END LOOP;

FOR result IN EXECUTE (tmp_stmt
|| ''old.'' || quote_ident(TG_ARGV[0])
|| '' = new.''
|| quote_ident(TG_ARGV[0])) LOOP
RETURN new;
END LOOP;

RAISE EXCEPTION ''Cannot modify % in %'', TG_ARGV[0], TG_RELNAME;
END IF;
END
';

I tried one or two other approaches in the dynamic statement, but
generally I get errors indicating that "new" and "old" can't be
referenced in this fashion:

ERROR: OLD used in query that is not in a rule

Is there a way to do what I want?