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

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