Delighted to discover Simon Evan's "Access Darts" today. This is the best highly accessible dart game I've ever played. You may be able to grab a free code from his blog if you're swift.
Not only is this game one-switch accessible, but it's also accessible for visually impaired players thanks to some excellent audio and assist modes. There's hopes for an update soon to include a free play mode, something I've long hoped for, which would make brilliant group games like Killler possible, and also would make it easier to play in virtual vs. real style. Really fantastic stuff.