Hi,
I have already given the potential mapping solution in one of the replies to TW [Aug 15, 2013 9:33 PM] in this long thread. This is a likely direction and you may not have a perfect solution as the requirement itself is not standard at least for SAP ERP.
Tell your SD and MM Guys to explore Third Party Customer Sales Order[not Vendor] Process for this. If they cant even help you / explore this, the only words I can think of for them - unprofessional or incompetent - and God save your SD & MM Business Users 
Have a happy solutioning.
I am dropping off of this discussion.
Thanks.