Another thought: I have a similar situation. I can manufacture a particular item in my US plant with a BOM that includes components I buy & any costs I care to associate with the BM. My std cost in this location is updated when I run costed BM update.
I can also buy this item from overseas already complete. When I do this, I purchase the item (tho mfg flag is set in item master, you CAN purchase a mfg item on a PO) into another location like CH for China. My CH location carries the std cost of the item on the PO. When I enter my sales order, I choose if I am selling from US loc (mfg in US from BM) or CH loc from China as a purchased product complete. My decision on which location sometimes has to do with which one has stock, sometimes customer requests "made in USA". Either way, cost comes to the order from iminvloc to the oeordlin & therefore the GL, POP orders are only affected if I need a POP to build in the US, otherwise costing comes from issue/receipt of PO in the CH location. In vanilla macola, you cannot change locations of line items on the fly, once saved to the order. You must delete & readd the item back in to draw from the alternate location. Peak BestOE allows a change to the location, however. As long as you are on cost to use when posting = "billed", all is fine if you use this source code enhancement, soon to be part of 7.6.300, I hear.
Sometimes we get so caught up in customers' unique needs, that we forget that macola can handle many combinations of variables generically. It can be worthwhile to visit the basics when presented with a new client request. Since macola already has some 7 million lines of programming code, trying to work within their framework, limited as it can be at times, can save many hours of future headaches by trying to "work around" the cobol code.