General inventory not showing on P&L by Customer

This is for a private events venue (weddings and the like). When we purchase general inventory (like wine), we don't know who the customer for that inventory is ultimately going to be. But I want the cost of the inventory to be associated with whichever customer does ultimately purchase it so that it appears on the P&L By Customer report. How can I do this? 

If I mark the original inventory purchase as billable, it requires me to select a specific customer to be billed (which, as I said, I don't know yet).  

If I don't mark the inventory item as billable when it is purchased, I can add it to a customer invoice and it adjusts the inventory valuation correctly. BUT it doesn't associate the cost of that inventory with the customer when I run the P&L. 

Going back to adjust the original bill for the purchased inventory after an event (and before I create the final customer invoice) and adjusting it to reflect what portion of the inventory was purchased for a specific customer is an extremeley tedious step that I need to avoid.