Perhaps creating a Business as Usual (BAU) Project and using the PSI to take the task info from Service Manager (7?) and populate the tasks.
To do this you would need SM to have duration, effort estimates (and actuals would be desirable) and a resource/task key field shared by PS and SM. That way each task could populate the BAU project and have the assignees effort estimates affect other projects resource availability.
I would think more about producing an SSRS exception report for project and line managers to show resources with conflicts and over utilisation due to BAU and project commitments.