Hi Tim,
We are glad to announce that on October 02nd we released a new generation of Web Apps that are much closer to the Mobile model, allowing developers to leverage client-side logic a lot more. You can read all about Reactive Web Apps on our forum post.
With the introduction of Reactive Web Apps, you can now change default input validation messages using Client Actions.

It's, of course, important to keep doing server-side validation, to prevent accepting invalid data because the client-side was tampered with.
Feel free to experiment with the new version and let us know if there's something we can improve.
Thanks again for sharing your idea!
Cheers!