Problem Solving Minimum Focus Distancing

[BruceBanner]

There are two lenses that I think could be significantly improved if they had a slight improvement with the minimum focus distance. The first is the XF50/1.0 which has 700mm as the minimum focus distance, the second is the IBELUX 40/0.85 which has a 750mm minimum focus distance. The 50/1 I do not have but the Kipon IBELUX 40/0.85 I do.

I recall one of my favorite togs (Barney Arthur) getting around the minimum focus distance of the Pentax 67 105/2.4 (1000mm minimum focus distance) by using extension tubes, he talks about it here;
 

I'm now pondering if any X Mount compatible extension tubes would be a viable work around to not getting macro results but just bringing that minimum focus distance constraint in a little to open up the lens creativity speaking.

Has anyone tried this before? Does it work, or do the MCEX-11 and 16 just 'zoom too much' for this purpose? What other options might we be able to use, to close that gap from 700-750mm down to something like 400mm?

TIA

BB