On second thought I guess there could be two allocations.
I.e. if state tax paid in 2019 and deducted on Schedule A in 2019 was incurred in 2018, it could be allocated to Net Investment Income based on 2018 income, even though the allocation will be subtracted from 2019 Net Investment Income.
And state tax incurred, paid, and deducted on Schedule A in 2019 would be allocated to Net Investment Income based on 2019 income.
I don’t see anything in the 8960 instructions to prevent that.
And this could work with the before-and-after method.