Any thoughts on the feasibility of this?
One thing you can't test in Rails is the result of Ajax requests in the context of the source document. The best you can do (as far as I'm aware) is test that the Ajax response is correct and hope the page gets updated. I started thinking...
WebKit has a Cocoa wrapper
You can manipulate Cocoa objects in Ruby via the RubyCocoa bridge
I've seen tools that can inspect the Safari DOM - presumably it would be easier if you controlled your own instance of WebKit.
I don't know much Cocoa myself - I haven't used it for a few years. I was hoping to run into someone here with more experience who could tell me if this is practical or just a crazy dream.