Postfix dynamic smtp_helo_name

This should work (not tried it, though):

Define a separate transport for the other domain and then route mail to this transport using sender_dependent_relayhost_maps option in main.cf.

Something like this:

In 'master.cf':

smtp-other      unix  -       -       n       -       -       smtp
     -o smtp_helo_name=my.other.helo.name

In 'main.cf':

sender_dependent_relayhost_maps = hash:/etc/postfix/relay_maps

In 'relay_maps':

my.other.domain    smtp-other:

And run 'postmap /etc/postfix/relay_maps'

Take look at Managing multiple Postfix instances on a single host, but be aware that multi-instance support is only available with Postfix version 2.6 and later.