Phill,
Antihalation dyes are located on the base side of the film. I don't use 35 mm so I can't say about that film but would imagine that it does have AH dyes too. It may be that the amount of dye to the amount of presoak water may not be as noticeable as sheet film. I presoak my sheet film for 5 minutes in tempered water bath. This does get rid of the AH dyes on FP4 in my experience. The pyro developer works on the emulsion side of the film so the AH dyes aren't involved in film developing. What happens is that if AH dyes are not removed is a build up in general film density that one needs to print through. Depending on the color of the dye, if not removed, may also affect the filtration on VC paper. Good luck.