This topic comes up from time to time and I think it would be a great way to do a number of things.
The utility of using EF lenses AND getting the full frame effect they were meant to give is more than just a passing nod to these optical treasures.
Focal reducers have been used in astronomy for more than 50 years. All the do is they make the resulting light cone steeper, the result is that exposure time is reduced, and giving a wider field of view.
For my 8" f/10 Intes Maksutov, it changes the optic from a 2000mm f/10 to a 1360mm f/6.3. This particular one works both with CCD cameras AND visually. There are some that will change this to a 1100mm f/5.5 or to 550mm f/3.3 but these are for CCD use only.