BPM Help - Pay Differential Based on Active Transactions

Wondering if anyone has written any BPMs or done any work around applying a differential for employees based on anything other than shifts? The end outcome that I am looking for is to have a differential applied only to an employee based on them being clocked into 3 activities. Currently we have a differential on the overnight shift and could create a new shift with this differential, relying on these specific employees to clock into that shift if they are going to work on 3 activities, but we would have to monitor to make sure they are not trying to cheat the system by always clocking into that shift. Would like to make it a little more reliable. At first thought, I figured any time an employee was clocked into 3 active transactions, the differential would apply, but would that require a new shift anyway? as differentials only apply to shifts? I also considered just writing a baq/dashboard so my accounting department could monitor, but any of these alternatives would require a bit of manual work and managing. Any ideas would be appreciated.

Eric