Payroll Balance Only for Union Assignments

Is there a way to tie any additional criteria (context or dimension) to payroll balances other than what is already available, such as union? And if not, what are others doing for this need?

We have payroll balances that are used specifically for union seniority hours. However since balances accumulate for all employees – and not just those in the union – it becomes an issue when an employee is non-union and moves into a union position, since they now already have existing hours in that balance.

We aren't aware of a current dimension that exists that would only start counting hours when they joined the union, which is indicated on the assignment record. When an employee goes from non-union to union, they stay in the same assignment. We're discussing seniority date functionality for this in combination with fast formula(s) but also feel like we don't know if that is the best route and if there are possibly other ways to achieve this. We'd prefer if there was a way to achieve this within the payroll balance directly.