Methyl violet will probably sensitize close to the same manner as ethyl violet. From the absorbence spectrum which I have seen published, ethyl violet sensitizes an emulsion to orange/red light rays as long as 6500 nm and methyl violet sensitizes to about 6400 nm. Both dyes improve red sensitizing when combined with a silver nitrate wash. I don't know why Yoshida lowered the temperature of his dye bath with ice, though I suspect that the lower temperature suppressed the index of absorbence of the dye and allowed a longer time in the dye bath which would cause smaller grains to be dyed.

Autochromes used ethyl violet as a sensitizing dye for orange/red light. If you notice that Autochromes do not record rich reds very well; most of the red tones lean to the pink/magenta. I suspect that this will be even more of a problem with methyl violet.