Chargeback vendor bills to multiple customers/jobs

As of 4.6, the ability to chargeback expenses incurred through a Vendor on a Bill, to a Customer on an Invoice, is limited to including line items on an invoice or not for a given Bill/Customer pairing. The UI label, "Default Chargeback Project" on a Bill implies that individual line items on the Bill could be assigned to non-default projects for the purposes of chargeback. Furthermore, the "Billable?" column accepts an "X" even when "Default Chargeback Project" is not set, consistent with the implication.

This request is to follow through with the implied functionality, enabling each line item on a single Bill to be charged back to a different customer/job.

Example use case: A carpenter visits the lumber yard. She buys 2x4s and framing nails for customer A's job, pine boards and jointing dowels for some built-in bookshelves for customer B, and some moulding and finishing nails for a customer C's job. All six items go on one Bill. Two line items are charged back to each of the three customers on three different Invoices.