Hi Anders,
Seems like something quite far from standard functionality available for ATP in sales.
The ATP works "only" at the date level, so you would have a hard time to implement this requirements.
In general, I find kind of strange this way of using the ATP because the concept in ATP is to confirm the pending sales at the document item level. Example: Out of 100 Units, only 50 units are confirmed.
Not time slot here.
Things to do IMHO:
Re-evaluate the possibilities of the standard ATP functionality (including rescheduling transaction V_V2) and its benefits. Evaluate again the customizing options and their impact. For example, what about activating only ATP for deliveries and not for sales orders? What about using replenishment lead time in the scope of check?
Do a brainstorming and evaluate other scenarios in SAP. What about small changes in the business process so you have a scenario more compatible with the way SAP works?
There is always a solution.
Be creative!
Sorry for the "consultant style" answer :-;
Best Regards,
Franck