You mean option to have helmet on, but off in dialogues?
I cant imagine this would be any high priority ... since you can hide it permanently, as DiDiDi said.
Get ready to imagine harder.... Larian's priorities are sometimes inscrutable. What DiDiDi was saying was that you can, indeed, already do exactly this - There are three settings for helmet display; always shown, always hidden, and hidden in dialogue/cutscenes; that's what the little speech bubble setting on the button is.