Well, for a different project (with a much wider scope, impact and budget) we re-determined the default billing type for specific documents in a user exit based on predefined criteria (there billing had to be performed without user interaction).
For the case which I previously mentioned, it made no sense to do that - only one customer and a few documents per year.
You are right, a different field could also be used to inform the billing department to use a different billing type, but the reality is that if you set no billing block to stop them, they will not check 
.