Annual working ratio - is there no option to prorate?

Summary:

Annual Working Weeks fields have been added to the assignment, but there is no obvious way to trigger proration in an event group

Content (please ensure you mask any confidential information):

We have "term time only" workers who are salaried but do not work every week of the year. A common practice in the UK is to spread their pay across the year, so basically their annual salary fast formula is modified to be multiplied by the Annual Working Ratio. This works, but there does not appear to be any event attached to these fields to trigger proration when the Annual Working Duration changes, or retro. We need this for their salary to calculate correctly.