Cast hstore type to bytea (and later to hex possibly)

Hello,

I would like to achieve something like the following:

COPY (select 'a=>x, b=>y'::hstore::bytea) TO STDOUT;

I have implemented an hstore value iterator that works with
pqxx::result::field.c_str() (which has its own binary serialisation format)
and I would like to reuse it to work with records that comes from stdin (in
hex format for example).

How can this be done?

Thank you,
Igor