OPM Costing - Percentage Overheads Based on Resource

Our overheads are calculated based on resource usage.  The % varies based on the department or resouce.  Unfortunately it does not appear that we can use resource, or resource class even as a priority when defining the overhead percentages.  Every priority is either Organization or Item related.  Our items are produced using multiple resources, in multiple departments and it does not look like I can manage the % overhead this way.  Any ideas would be greatly appreciated.

We are on Standard Costing.