I would like this option also.
"I think that the Sage 50 functionality was added from 'higher' versions like Sage 100. I've noticed that the data structure for Orders' Projects stores the allocated project ID and percentage separately for each line, even though the program interface only allows choosing one project for the entire Order.
This looks to me like a deliberate limitation of the Orders in Sage 50. But in case it's just not finished yet, I'll add my vote to Trevor's."