Cast timestamptz to/from integer?

I work a lot with Unix times as integers, but would like to store them
in Postgres as 'timestamp(0) with time zone' for convenience and
readability. Unfortunately the syntax to translate between the two is
a little cumbersome, so I'm looking at hiding it away behind a
function - or a cast. However, the CREATE CAST docs say that I have to
be the owner of either the source or target types, which presumably
means I can't define a cast from timestamptz to int.

Is there a workaround for this?

Chris Angelico