Custom Operators Cannot be Found for Composite Type Values

Hackers,

I’m doing some development with the new JSON type (actually, Andrew’s backport to 9.1) and needed to do some very basic
equivalencetesting. So I created a custom operator: 
   CREATE OR REPLACE FUNCTION json_eq(       json,       json   ) RETURNS BOOLEAN LANGUAGE SQL STRICT IMMUTABLE AS $$
   SELECT $1::text = $2::text;   $$; 
   CREATE OPERATOR = (       LEFTARG   = json,       RIGHTARG  = json,       PROCEDURE = json_eq   );

With this in place, these work:
    SELECT '{}'::json = '{}'::json;    SELECT ROW('{}'::json) = ROW('{}'::json);

However this does not:
   create type ajson AS (a json);   SELECT ROW('{}'::json)::ajson = ROW('{}'::json)::ajson;

That last line emits an error:
   ERROR:  could not identify an equality operator for type json

To which my response was: WTF? Is this expected behavior? Is there something about custom operators that they can’t be
usedto compare the values of values in composite types? 

I’ve worked around it by writing a separate operator to compare ajson types using
   SELECT $1::text = $2::text

But it’s a bit annoying.

Thanks,

David