I hope I was clear that if doing what I think is silly pleases you then by all means do what I think as silly and be happy. I'll be happy for you. No sarcasm intended.
I haven't said that the joy of tinkering is worth a lot. It is, and I've known it m'self. Re that, see
http://www.galerie-photo.com/telechargement/dan-fromm-6x9-lenses-v2-2011-03-29.pdf and
http://www.galerie-photo.com/baby-bertha-6x9-en.html
There are fastish lenses for 6x6 and 6x9. 80/2.8 and 100/2.8 Planars, both of which cover 6x9; 95/2.8 Heligon; 80/2.8 and 100/2.8 Xenotars, of which the 80 doesn't cover 6x9. I use a 100/2 TTH Anastigmat, ex-Vinten F95, and have a 100/2.5 Uran-27 that I don't use. Both are adapted to my 2x3 Speed Graphic. If I wanted one, there's the 120/2 SFOM for the SFOM 680/681. There's a variety of 150/2.8 lenses out there. There's a 180/2.8 Sonnar. There are 210 and longer f/4.5 Tessars. Y'r appeal to non-existence of fastish lenses that cover at least 2x3 is full of eels.
Retrofocus wide angles are another matter. The 1.75"/2.8 Elcan barely covers 6x6, is scarce, not all that good, and the shortest lens I know of that can be used easily on a 2x3 Speed Graphic. That aside, there are retrofocus lenses for the Pentax 67 (shorter flange distance than the RB/Z), RB/Z and, as has been mentioned, Fuji GX 680. For the six or seven (surely I exaggerate) people in the world still using Arca-Swiss Reflexes and Makiflexes and Pecoflexes they may be attractive. For the rest of us there are many alternatives.