Preventing a Sales Order from consuming forecast:

We have this situation where using a custom hook we currently exclude some customers from driving the plan (set demand_visible = 'N', available_to_mrp = 'N' on msc_st_sales_orders) so it doesnt drive MRP since we only drive them by forecast.

In certain situations though when we dont have a forecast and have some drop in demand in the form of a Sales Order we would like to drive that Sales Order to generate demand but don't want to consume the forecast that exists. We will possibly use a new ship to or something like that so the custom hook doesn't block it but how can I prevent a Sales Order in MRP not consume existing forecast. I understand that I could populate a dummy demand class on the SO and not have any demand class on the forecast but according