thnx for your comments on this.
In my case we do have a storage type which only needs to be accessed for a certain customer as well. The reason for this is the customer requires a different box sice which is it's standard. As this is a big customer with lots of orders there is a whole storage type just for this customer. Also this is the only storage type which fits this special box size.
If this makes sence or not is another topic right here, but this can not be changed in matter of efficiancy and customer requirement.
Only Solution is currently to implement a Badi, which is not a big deal.
Badi: /SCWM/EX_CORE_RMS_DETERMINE
br,
A