implicit CAST on CSV COPY FROM

Hi

I have a set of CSV data that I'm importing containing dates stored as INT
values (eg 20150402). The value 0 represents a null date in this format.

I've created a function and cast that (ab)uses the system text::date cast:

CREATE FUNCTION to_date(integer) RETURNS date AS $$SELECT CASE WHEN $1<=0
THEN NULL ELSE CONCAT(($1/10000)::text, '-', (($1/100)%100)::text, '-',
($1%100)::text)::date END$$ LANGUAGE SQL;

CREATE CAST (integer AS date) WITH FUNCTION to_date(integer) AS IMPLICIT;

This works fine if I

CREATE TABLE mytable (dt date NULL);

INSERT INTO mytable (dt) VALUES (0);
mydb=# INSERT INTO mytable (dt) VALUES (0);
INSERT 0 1
Time: 1.562 ms

but if I use

COPY mytable (dt) FROM STDIN WITH (FORMAT csv, HEADER false);
0
\.

it fails.

ERROR: date/time field value out of range: "0"
HINT: Perhaps you need a different "datestyle" setting.
CONTEXT: COPY mytable, line 1, column dt: "0"

Is there a trick I can use to get COPY FROM to use my cast? Is it somehow
treating all the CSV values as strings and then trying to cast to the
target type? I tried creating similar CASTs for text, char and varchar to
call my to_date function but none of them made a difference.

Thanks!

Geoff