I am usually more likely to remove stiffeners in bags than to add them.

I have not used Carradice bags, but typically pack any bags I use so that they need no further stiffening. I normally roll, compress, or otherwise pack stuff so it takes as little space as possible then size bags to just barely work when I have some extra food and/or water. If It doesn't completely fill a bag well enough to hold its shape, I can always pack clothing or sleeping bag looser so that it takes up a bit more space. Failing that putting a little air in my inflatable pillow or sleeping pad would do the trick.

I much prefer to just size all bags on the bike so they are always fairly full. On the relatively rare occasion that I need more space I take little backpack that will carry a few pounds of overflow for that long dry stretch. That or I stuff a few liters of water and some of the food in my jersey pockets.

I'd think that with a rack under the bag it would be even less of a problem.