This looks like it might make a good springboard for a Metered Billing app. I wish I knew how to code, because I would look into forking this myself, but it never hurts to suggest someone with an idea.
Off the top of my head, the two markets I can see Metered Billing used are Utility Companies (billing customer per KwH, Water, Gas, etc) and Managed Print Support.
To elaborate on Managed Print: some customers, who use office printers and copiers, have their equipment under contract, where they pay a monthly rate that is usually dependent on the number of pages their equipment prints (For example: the customer may be charged $.015 per page printed, and if they print 8,200 pages in a month, then they would receive a bill for $123 for that month), but will never receive individual bills for Supplies, Parts or Service.
There are commercial ERP solutions on the market, e-Automate and OMD probably being two of the largest, but they are incredibly expensive and cumbersome to setup. I would imagine that if someone developed similar functionality into ERPNext, it would provide them with a HUGE advantage in the open source market, since many of the small dealers are stuck using off the shelf software like Quickbooks, which doesn’t provide anything remotely close to a designated ERP.