converting E'C:\\something' to bytea

Hello,

Is there any way of casting (reinterpreting) a varchar/text field
containing arbitrary backslashes to bytea without making an escaped
copy of the varchar/text first?  In the examples below I am using a
constant E'...' for clarity, the value normally comes from a
varchar/text column in a table but the end behaviour is the same.

E.g.:

1) SELECT E'C:\\something'::bytea
    ERROR:  invalid input syntax for type bytea
    --> essentially like calling decode(); bad in this case because of
the naked backslash!

2) SELECT replace(E'C:\\something', E'\\', E'\\\\')::bytea
    --> works OK, but bad performance-wise because needed to make an
escaped copy of the string which is inefficient

3) CREATE DOMAIN my_varlena AS text;
    CREATE CAST (my_varlena AS bytea) WITHOUT FUNCTION;
    SELECT E'C:\\something'::my_varlena::bytea
    ERROR:  invalid input syntax for type bytea
    --> WHY?

Thanks,
V.