It isn't just the possibility of the mirror impacting, it is also the size of the image circle projected. While an extension tube moves the lens away (and thus will make the image circle a little bigger in some cases) it's still going to have all the extension tube caveats. You would likely also have to shoot full manual as the camera probably isn't going to detect or allow electrical connection to an EF-s lens with which it isn't compatible.

Even if the extension tube fakes it for you, auto-focus with an extension tube can be pretty dicey. As Michael Nielsen mentioned, you will almost certainly lose the ability to focus to infinity (though you will also reduce the minimum focus distance).

You would also want to make sure it was a long enough extension tube before trying it. A short extension tube may still not clear the mirror if the optics extend out of the back of the lens at some part of the zoom.