Hello Scott,
As you mentioned the requirement is quite unique.
I assume the customer charge is a fixed charge based on days. If that's the case can you post these charges at the CA level instead of contract. You can maintain separate ADID at the CA and Contract level. Clunky solution.
Another option will be to use different main transaction for customer charge.
Keep us posted on the solution route you take, as it is an interesting topic and something which customers might demand in the future.
Regards,
Rahul