as usual, the problem's seldom where you 1st look. *NO* need for _any_ changes to queries.
the source of the problem was my local postfix submission instance's config -- connected to, as submission_relay_host, by dovecot. it was doing exactly what I'd told it to do ... NOT expanding alias address mappings. ensuring that no_address_mappings is removed from receive_override_options= , whether in main.cf, or in the instance's config in master.cf, does the trick. i.e., pebkac.