Hi Lucas,
Why don't you use the make to order strategy like 20?
So that system will not consider the unrestricted or customer specific stock.
Also you can exclude the storage location from MRP OMIR select the MRP indicator as 1 for that storage location.
Or in material master if you maintain SL MRP = 1- exclude SL from MRP in MRP4 view, then system will not consider the stock for any calculation.
Also you can use the BAdI MD_CHANGE_MRP_DATA and use method CHANGE_MDPSX_MARD as per the documentation BAdI method CHANGE_MDPSX_MARD is called when storage location stocks are imported from the database. After the standard logic has run, you can change the corresponding entry in table MDPSX for the individual storage location types via the specified structure.
Regards,
R.Brahmankar