It is possible to get the value from the new field.

How to do it: make sure you have an object of sale_order. When using the browse method, you can use a dot (.) to access the value. Check the various .py files.

Example (from sale.py, in folder /addons/sale):

'currency_id': self.pool.get('product.pricelist').browse(cr, uid, pricelist_id, context=context).currency_id.id

This will store the value of the ID of the currency of the product.pricelist in the variable currency_id