Ideally yes, Window.js should make it possible (and easy!) to build fully accessible programs. I don't have much experience in this area though, so I'd have to look for help building this :-)
There's AccessKit which would work great if we were a few years in the future and they'd finished their bindings and non-windows implementations. Other than that I don't know of anything cross-platform other than Qt which is rather large.
Ideally yes, Window.js should make it possible (and easy!) to build fully accessible programs. I don't have much experience in this area though, so I'd have to look for help building this :-)