Since this isn't a supported feature, I can point you in the correct direction, but you'll have to code it on your own.
You will have to create a function that will set the display of the value. Then you would call this function in the onchange of the select box. The function will need to have the value of the selected option to be passed in to the function as a parameter and then it will use a recordset to find the sku value of the option. After that it will set the display to the correct sku.