It occurs to me that an alternative to starch, would be one of the fine silicas
or other such fillers that are used to reduce the gloss of surface coatings.>
This formula I mentioned is rather old but was used until some years ago with
good succes. I haven't tried other silicas but at the moment I see no reason why
it shouldn't work. No idea how much you would have to add of those substances,
but probably less then for an ordinary semi-matte silver bromide emulsion.
Good luck and please let me know how it works.
Klaus