I did a week with a small 1L Camelbak, which I'd guess weighs 4 pounds with a little stuff in it. It worked out OK, but I don't think I would want to tour with a larger, heavier, bulkier backpack.

I have one of those Deuter backpacks. The pack is kept off your back about an inch or so by an internal frame. However, it also uses a big mesh panel that maintains the separation. So far I haven't found it to be much cooler on a bike than a small channeled Camelbak.

I also think it's a tad silly to fuss over 1 pound, especially in a touring context. If you were climbing Alpe d'Huze (8 miles of a consistent 8% grade) @ 200 watts (i.e. climbing at 6mph), adding 1 pound would add less than a minute out of 1 hour and 20 minutes of climbing. I don't see why dropping a pound will be worth the additional discomfort.

Using a backpack certainly won't kill you. I'm thinking that you won't want to do it twice.