This certainly explains why you are going through the SD route, I can understand this approach.
I have no solution off the shelf. Let us a bit brainstorm.
you do not want to bill and you cannot receive the bill within one company.
you need the sales order for a make to order production according to the variant classification.
why not just doing a transfer posting from sales order stock to own stock when the production is finished and rejecting this sales order?
This would avoid the need to do a goods issue and to bill the material.
I do not see a need for 2 material types either. a material type defines common attributes for a material and controls which number is to be used, just get rid of thinking about a meaning behind names like ERSA or FERT. Call it e.g. ZMAT and less people want complain or discuss about material types.
I have no idea how you could MRP let determine which variant has to be produced. If you have a reliable basis for such info, then you can probably do something with BAPIs. Not sure if you would really need PR and PO, or better directly create a sales order.
Maybe the folks from SAP ERP Manufacturing - Production Planning (SAP PP) or the guys from SAP ERP PLM - Classification and Variant Configuration
have an idea on that issue.