Figured it out -- IA Schedule CC. It is under Composite Credits, not pass-through withholding and it applies even though TurboTax limits applicability to Partnerships and S Corporations and not Trusts/Estates.
TurboTax still does not list Iowa for us as a Nonresident return -- actually blank on the summary screen as well, but all the forms seemingly printed out.
The Iowa updates still haven't triggered Schedule CC -- have to go into Forms and manual request and complete.