Thanks for the comments.
The breaker is is not being backfed so 408.36(F) wouldn't apply.
I know all service conductors require a disconnecting means and all service conductors require overload protection.
However in this case, there are no loads that will overload the unused leg and there is nothing actually being disconnected even with a breaker installed on that unused pole.
The code permits an MLO to be used as service equipment under certain conditions. I just not sure what the correct solution for this scenerio is.
