This is the first year I've had to address this, but what seems to be working is to enter the disallowed deduction/safe harbor amount directly in the State column of line 18a Other Withholding of the "Tax Payments" Worksheet (under Forms). Then for the "St" entry to the left, I'm entering "SH" for Safe Harbor. (I think you could use any two letters that aren't the abbreviation for a real state.). Turbotax then treats this amount on the federal forms as state tax withheld for the SALT deduction (included on Line 5a of the federal Schedule A), but it does not include this as withholding on the state income tax forms (apparently because it doesn't know which state to associate it with). I'm not finding this to create any problems or miscalculations.
