Angelo,
The only odd thing about your scenario is your statement
2)Sales orders and work orders have to consume forecasts.
It makes no sense to me to have a supply element (work order) consume a demand element (forecast).
Other than that, no special transaction required. Standard Scheduling agreement technology will suffice.
Your term for 'order book' is not a SAP term; and it means many different things to different people. If you are just talking about BI/Statistical reports, they can be crafted to include or exclude just about any type of supply or demand elements, that is merely a design criteria and should not influence your planning decisions..Likewise, any type of sales document item can be designed to transfer requirements to MRP, or not - it is all configurable.
SAP has standard IDOC technology to update and manage Sales Scheduling agreements, both for forecasts, and also for customers who send releases against forecasts. SAP also supports most EDI mapping software; or you can use SAP's own middleware. If you insist upon having discreet orders separate from your SAs, you can use LZM type SAs consumed by TAM orders, which are designed to work together.
In short, from what you have stated so far, (except consumption by work orders) I see no reason to use any custom programs. Speak to your local SD expert about these matters. If you have never migrated Sales and Distribution from a legacy system to a SAP system before, I strongly recommend that you engage expert resources to assist you; this is not a trivial matter. Money spent here for the right resources will generate huge savings going forward, in both your implementation costs and also your ongoing maintenance costs.
Best Regards,
DB49