I did some rough estimates of how much transmission would improve if just the prism glass in the FL were replaced with one of the new HT glasses that Zeiss refers to as a "prism glass" (N-SK2HT, since we know N-B7HT wouldn't be suitable). This is based on 75mm travel through glass in the prism and the old prism glass being the non-HT version, N-SK2.
400nm, +1.8%
450nm, +2.7%
500nm, +1.5%
550nm, +0.6%
600nm, +0.6%
650nm, +0.6%
A nice boost in the blue, but not much help at the red end.
Edit: FWIW, If the current prism glass is BAK4, the transmission improvement would be totally insignificant at wavelengths of 500nm or longer (+0.3%), but would be around +3% at 400nm.