change JSON serialization for BIGINT?

Is there a way to have Postgres serialize BIGINT as a string rather than number in JSON?  By default it does this:

select row_to_json(row(500::bigint));
 row_to_json
-------------
 {"f1":500}

But I want it to do this (note that "500" is quoted):

select row_to_json(row(500::bigint));
 row_to_json
-------------
 {"f1":"500"}

I tried doing this, but it has no effect:

CREATE or replace FUNCTION bigintJson(bigint) RETURNS json
    AS 'select $1::text::json;'
    LANGUAGE SQL IMMUTABLE RETURNS NULL ON NULL INPUT;

create cast (bigint as json) with function bigintJson(bigint) as implicit;

Thanks for any guidance.

--
Tim McLaughlin