I'm not sure that there's a definitive answer to your question. You could take your lead from the behaviour of others who tend to keep their valuables and lightweight items including snacks in the bar bag.
With a suitably large saddlebaq - assuming that you are using B&B and hostels/hotels you shouldn't need to overload the bar bag.
Have you looked at the carradice web site
www.carradice.co.uk and spa cycles at
www.spacycles.co.uk ? Between the two you should be able to find suitable baggage.