Here's different suggestion. I personally don't like the idea of having the laptop on the bike itself. It's subject to a lot more shock and vibration then when on your person.
I do sympathize with your situation though and I've found that the weight of the load on my back and shoulders is only the part of the problem. Bulk makes a difference too. A large and stuffed messenger bag gets uncomfortable after awhile even if the stuff in it isn't that heavy. Moderately heavy items alone like a smaller laptop aren't that bad if kept close to the body.
Something that might help is to keep carrying the laptop in a messenger back or backpack and then put your clothes and other items in a trunk or pannier. It gets a little weight off you and a lot of the bulk.