Which of all possible billing relevances you refer to - order-related billing has many variations. 
In one of my previous projects we used mainly order-related billing of delivery quantity in the standard OTC business process and it worked quite well. We had some developments such as a VOFM routine to disallow billing of order item without a delivery and GI (similar to the cash sales routine), but they were implemented not because of intercompany processes.
What is the problem that you are trying to solve by switching to order-related billing process?
Do you need also to execute intercompany billing before billing the end customer with collective processing? If you use suchs approach I would suggest to test very well the effect of the user exit before deploying the changes to the productive system.