There is no other way.
(Well, of course, it is possible, but please trust me - it's too
complicated.)
Too complicated!?!? What's complicated about using tar in the root
``/'' directory? Oh, yea, it's the meta package control that can't be
easily duplicated. The postinstall scripts would need to be manually
executed. The setup control data would need to be populated; but, if
you don't use setup to upgrade that shouldn't matter. Other,
automagical stuff that is executed by setup would be ignored or even
unknown.