Would it be feasible to reverse a 1.4x teleconverter's optics into a cross-system lens mount adapter & have it function as a focal reducer?
For example, like this:
Camera hacked adapter Lens with body ← w/ → longer reverse TC optics registration
This would entail cannibalizing a 1.4x TC & mounting its optics backwards into a mount adapter.
Could it also sharpen the center of the field a scosche, by forcing optical concentration of the field, as well as gain a partial stop by intensifying the field?
If this is feasible, it might also be feasible for Tx/T2/T 135/35mm mount lenses, assuming they were engineered with extra field to accommodate Nikon's long lens registration, when adapting to far shorter registration length/flange distance systems (basically anyone shorter than Leica R or Nikon F).
Plasticine / JB-Weld not included.
"Poor Man's Focal Reducer" at mflenses.com (a "manual-focus" lense afficionado site):
Entails some trial & error: