I know the history of hand flown HUD-based CAT II and III approaches but doing them in a FAR 121 operation is far different than in a corporate environment. I'd bet ALL of them in the YS were 121 operators.
Just maintaining currency would be a challenge in 91, plus the training burden, the aircraft maintenance demand for something used seldomly. I have 29 years of C-5 ops and an additional 15 of corporate--RVR 600 take-offs equal zero and CAT II landings, 1 in the C-5, none in the corporates. Might be useful to a Zurich operator, tho.
GF