I would have the vet or chiro check her over for soreness and/or places where she might need some adjustment. If she is sound and pain free, then I see no reason why she couldn't be used for light riding. The biggest problem you'll run into will be getting a saddle to fit her well so that it doesn't pinch or put too much pressure on any one particular area of her back. There are specialty saddle pads that you can buy that have shims that you can put in to fill in the areas where you need more padding, like the middle of her back where the dip is the lowest. It will likely be expensive to get the saddle to fit her well enough to not cause problems.
If she doesn't show pain when your riding her then don't stop. Have the vet look her over and make sure she has proper fit tack. And your good to go imo.
If you know how to ride bareback my vet said that's better for there backs. I do it with my mare with a severe sway worse then your mares. But we only ride our girl 20 minutes 3 times a month to get her mind moving again :)
I don't think she should have any problem with light riding. There is a mare at my local gymkhana that has a worse sway back then yours and she is ridden by a younger girl and they do perfectly fine :]
As long as she's not hurting, there is no reason not to ride her. Really, her swayback isn't half a bad as some that I've seen.
Like smrobs said though, saddle fit can get expensive, and if you don't do it right, you can damage your horse's back. We have a 28 year old mare with a swayback at about the same severity as your girl's, that we still use for lessons, who- for a while, we would just tack her with two saddle pads and a normal saddle. Within a few months though she was acting up and just over-all being pissy, a sure sign that she wasn't feeling good, considering that she's a beautifully trained WP mare. Got a chiro and saddle fitter out, and it ended up costing us about $350, while we could of saved a good bit of that by just fitting her in the first place.
What works good for her though, is a Cashel Wool Swayback Saddle-Pad. After an adjustment and the new saddlepad, as well as a fairly deep gulletted, properly fit saddle- and she's back to her happy, willing self. Have your saddle fitter or chiro recommend a saddle or a pad for you, and don't do what we did!
Just be sensitive and watchful for signs of discomfort, and condition her properly. She's got a good 5-10 years of trail riding in there!
Two pictures of our gal- Puddin'. Completely unflattering first picture, but its the only one I have of her untacked. No she's not foundering, she's getting up after a roll ;)