Sorry I have not gotten back to this.
In order to use a back fed breaker as the required building disconnecting means the panel must be listed as service equipment with a back fed breaker.
Tom I am not deputing that but what article requires that? 110.3(B)?
Is there a panel that can not be backfed and why?
It is true that you can forget about the backfed main with 6 breakers or less but only if it is not a lighting and appliance branch circuit panel board. 408.16(A)
Buck I do not see how this changes anything, the overcurrent protection can be (and usually is) taken care of at the panel the feed originates from.shortcircuit
What I was trying to get at but was in to much of a hurry to get across before.
1)You could install a main breaker panel.
2)You could backfed a main lug panel, with a secured breaker and following Tom's post.
3)If you can live with 6 circuits or less you can install a main lug panel, fed it from the lugs and use the 6 or less breakers as the disconnecting means. They will not have to be secured because they are not back fed.
Remember the 225.31 does not require the panel to be shut down only the conductors.