I've found that ' Steamin Demon BT' at the NavWeaps discussion boards supplies some of the information that I've been looking for:
Re: Scott's Arguments for the Iowa Class Battleships
Re: The utility of steam propulsion in the 21st Century
Ignore what he says in the second post about disagreeing with Dick L, if you read the post he's replying to he's actually agreeing with him (that the utility of an Iowa steam plant is underrated).
From this, I assume (apologies to Tony Randall) that it'd be possible to steam with a crew that could fully man the propulsion stations only during general quarters; and given that all spaces wouldn't take casualties, you could probably steam with less than that.