Automatically select price list on invoice based on user-defined field ?

We have a case in which the company has several price lists for each customer. The price list to be used for each invoice is determined on the basis of the customer's end customer.

We were planning on adding a user-defined structured field 'end customer' and a formatted search on the pricelist field that would get its value from a query. However, the problem is that the pricelist field is not visible on the invoice form; it can only be accessed by opening the 'settings' form for the invoice. Now, if I try to add a formatted search query to the pricelist field on the settings form, the 'auto-refresh when fields change' option only offers fields that are related to the master data records, not those on the invoice form. As a result, it seems we cannot update the price list setting on the invoice based on the value in a user-defined field.

Are we doomed to create a separate BP record for each Customer/End Customer combination or is there a better solution to this problem