Nikonos 15mm + macro extension tubes for wide angle macro

Has anyone ever tried the macro extension rings on the Nikonos 15mm or 20mm lenses for wide angle macro? Can it even work theoretically?

Extension rings are effectively just spacers, so if you could find the right size extension, no reason it wouldn't work.  The catch is you only need a very short extension tube as the focal length of the lens is so short.  I tried plugging some numbers into a macro calculator and came up with a 5mm extension to get half life size. 

That is in air and I'm assuming the Nikonos 15 would  focus on a virtual image like a lens behind a dome port and throw those calculations out quite a bit.  The Nikonos 15mm is effectively a 20mm nikon wide angle lens behind an optimised dome as I understand it. The virtual image is very close to a small dome and is quite compressed so the lens only has to focus a very small amount closer to focus on closer objects - this likely would be the catch - getting a small enough extension to focus closer but not lose middle distances.

There were extension rings made (bij Sea&Sea I believe?) in the past so I would give it a try, but like Chris said, the focus point would quickly become very close I think. Although stopping down (especially on the wider lenses) should help with depth of field a bit.
Curious to try this myself too.