use of inet_protocols= option in policy maps?

[PGNet Dev]

i've a relay def'd in master.cf

        relay-test  unix  -  -  n  -  -  smtp
          ...
          -o 
smtp_tls_policy_maps=${def_db_type}:${conf_dir}/test/relay_tls_policy

entries is 'relay_tls_policy' take usual form, per 
http://www.postfix.org/TLS_README.html#client_tls_policy, e.g.

        [hostname.example.com]:25     secure    match=hostname.example.com
        ...

for the relay, i can lockdown transport to a specific inet protocol,

        -o inet_protocols=ipv4

but that applies to all hosts in the map

i haven't found (yet) a doc'd example of spec'ing per-host inet_protocols in 
maps.

is it doable? if so, what doc/example is relevant?

thx