Hi BC
If BOM is the same in different plants, you can create a "plant independant bom", and assing to the plant with tcode CS07. Some years ago i face the same problem, we need to create about 250 plant for every farm in a poultry farming proyect, cos the customer want detailed cost per farm. If we define a storage location per farm in a plant, the cost of the raw and the fert are mixed within the plant so the company could accomplisht the desired behavior. In adition to the farms, we have about 10 production plants for incubation, balanced food, meet procesing, and another 20 for sales areas (about 10 storage locations representing a store per plant) This proyect also has ML and the costing process was a bit slow (about 4/5 hours), in spite it dont have too much more materials (about 2000). In this i did the PP stuff.
About routing, i guess PP boys must create work centers and routing independant they work with storage locations or plants, because the equipment/machines are different and also quite sure thy have different cost centers. If they insist, they can also create Routing independant of the plant.
+ About work with product cost collectors (REM or PP Orders), have in mind you coudnt work with co-products costing.
Arturo.