Ben
Have you considered setting the Customer Number field directly, without using the lookup? You would have to know the Customer Number prior to opening the form, but you could just simply pass in that value to the field, and then run the Customer Number change script.
Sanscript example:
open form RM_Customer_Maintenance;
set 'Customer Number' of window RM_Customer_Maintenance of form RM_Customer_Maintenance to 'XXX'; (your value or field containing the customer number.)
run script 'Customer Number' of window RM_Customer_Maintenance of form RM_Customer_Maintenance;
Hope this helps,
Aaron