Hmm, you could do a PO for vendor A, drop shipping to vendor B who you also have a regular PO for the finished item. You could then do a landed cost for vendor A portion on the receipt for vendor B PO. You would have to make sure the landed cost and invoice for vendor A PO both hit the appropriate accounts.
Very wild workaround but I never said it would be pretty. If it doesn't make sense, forgive me as I've been standing in front of about a dozen people all day shooting around ideas like this. There is USUALLY a workaround for everything. The question is, is it worth it?