IR/ISO dropship from an external Supplier

We have a plant in OU1 who will create an IR to a plant in OU2.  Plant in OU2 actually contracts with a manufacturer(they handle capacity overflow/Geographic preference) to supply the product to the plant in OU1.  Question is how to handle this IR/ISO process to create an external dropship?  Any ideas are greatly appreciated.  I tried to trick it into a BACK to Back order from the IR/ISO process.  Made the Item an ATO item, Supplier sourced, Buy item - no inventory in OU2 plant.  It is on a BPA, ASL, and SR