I have the same issue, until now I have not solved it yet. Any help please

You could always use

self.pool.get('sale.order').write(.....)

But I've been told that this is neither well apllicable nor good style.

For better help please give more details on what you are trying to do.

Have you thought about using an onchange function in sale_order on sale_order_lines?
If product_id changes the sale_order_line might change which triggers the onchange that has direct acces to the sale_order fields.