This feature has been implemented. In order for it to work there are a few steps that are needed.
First of all the purchased/Outside Service must be coded as a Job Costed item on the item site record.
The Item site must also be setup to create a PO for all WO demand for that item.
So when a WO is released which calls for the Job Costed item a PO is created for that item.
When that PO item is received, it is then immediately issued to the work order.
The consequence of this is that you should not have inventory for this item. The cost for the item will come from the PO price.