Custom type literal conversion

I would like to create a new type for version strings that sorts
numerically. The composite type below was quick to write and does not
require superuser privileges. However, it doesn't respond to type casts the
way I'd like.

Is there a way to implement this type's literal conversion without
resorting to a base type?

Thanks
-- Chris

===

=> CREATE TYPE myversion AS (version integer[], build varchar);
CREATE TYPE
=> CREATE OR REPLACE FUNCTION myversion(text)
-> RETURNS myversion AS $$
$> SELECT
$> string_to_array(split_part($1, '+', 1), '.')::integer[],
$> split_part($1, '+', 2);
$> $$ LANGUAGE SQL IMMUTABLE STRICT;
CREATE FUNCTION
=> SELECT myversion('1.0.9+jenkins');
myversion
---------------------
("{1,0,9}",jenkins)
(1 row)

=> SELECT '1.0.9+jenkins'::myversion;
ERROR: malformed record literal: "1.0.9+jenkins"
LINE 1: SELECT '1.0.9+jenkins'::myversion;
^
DETAIL: Missing left parenthesis.