Hi Caroline,
If you are passing the data to ECC or POS DM through a middleware or any upload program, it is better to keep this validation in the middleware or an interface program itself, so from middleware itself the data will be segregated based on brand.
We did similar types of validations in SAP XI which was integrating our POS to POS DM.
If you are still looking for an exit in ECC for Inbound will give you in sometime.
I hope this will help.
Regards,
Amit