I owned an old girl (you can view her pictures here - her name is Princess) had a really swayed back. Her trot was a little rough, but her canter/gallop was freaking amazing. Just like any straight backed horse, they have their own feel. Age just sometimes gets to a horse's back when ridden a lot, or are bred, or other nonsense. But you already know that. :P
It never really affected her except during her low-weight times when she couldn't eat well. Her cushings didn't help either. So, make sure your girl stays fairly decent in weight, otherwise her anus will sink in and she'll poop all over herself and that can cause problems - not just with cleaning, but as you know, they can't wipe so the bacteria will be a health hazard.
Hope that helps to an extent.