I sometimes carry my 13" Macbook in my back rollers. I usually just put it in its case, wrap a towel/jeans/whatever around it to prevent it from bouncing around, and ride. Because of the heel taper on the back rollers, you're going to have somewhat limited space for a 15" laptop (side to side). My guess is that it should fit with even with a bit of padding. The hardware sticks into the bags a bit on the rack side of the back rollers (at least the classics) and you want to somehow insulate your laptop from this.

If you think you can swing it, a netbook would be a much better idea for touring. It would save you a lot of space, risk, and weight. Netbooks can usually be had for less than the cost of the Applecare warranty for your laptop and will probably do more to extend its life.

Also, it may be worth mentioning that Ortlieb makes an SLR insert for their handlebar bag.