Dynamically update NEW columns in plpgsql trigger

I need to dynamically update NEW columns. Ive been inserting the NEW
values into a temp table, updating them, then passing the temp table
values back to NEW (is there a better way?). Ive had success with this
method unless there is a null value...

EXECUTE 'CREATE TEMP TABLE new AS SELECT $1.*' USING NEW;
EXECUTE 'UPDATE new SET ' || dyn_col_name || ' = 1';
EXECUTE 'SELECT * FROM new' INTO NEW;
EXECUTE 'DROP TABLE new';

This last line...
EXECUTE 'SELECT $1.' || dyn_col_name INTO val USING NEW;

gives the ERROR: could not identify column "col_one" in record data
type.

However RAISE EXCEPTION '%',NEW.col_one;
returns "1" correctly.

If col_one does does not start out as a null value, then everything
works. Why does the passing from temp table back to NEW lose the USING
functionality?