Thanks for the link. I agree with the one comment posted to the review - that was a pretty bad misrepresentation. As Dr. Croubie points out, the rear element of the lens is right at the back. I think he mis-typed when stating it's at 300mm - I think it's at 70mm and the rear element moves further 'in' (away from the lens mount) as you zoom to 300mm - at least, that's how most extending zooms behave (except the 24-70mm which uses a reversed design). So, it's 'compatible' at longer focal lengths, but not 'compatible' (= lens damage) at shorter focal lengths.
An apt theory.. since it will be used for longer lengths anyway, it could work.
Dr. Croubie: Thanks for the extra pics, I would not have understood it well without them.
Is lens damage really a concern? I mean there's plastic hitting glass, so unless someone is careless, I would think the risk is limited if used with the knowledge that the floating lens hunts, No AF to be used and should keep the zoom at 300mm?... I guess not worth buying a brand new 2x over the 70-300mm unless one had one lying around....