no, you wont be able to use the $WA_redirectURL since this is set before the record is inserted.
you need to leave that blank, then do the redirect manually after the record is inserted to be able to use the session.
keep in mind PHP is executed from top to bottom.
where the $WA_redirectURL is set before the code to insert the record, the session does not exist when it gets set, it only exists after the record is inserted.