I've not seen the Klingon Forehead Arc, but the show hogtied itself putting a bumpy Klingon in "Broken Bow" in the first place. If they'd started with smoothies, there would have been no need to explain STTMP Klingons, and it could have been left at DS9's "We do not discuss it with outsiders." It wasn't an issue in Voyager. It could have just been an unexplained mystery which didn't really need to be solved.
It was just another problem I had with the series, which tried to put too much emphasis on 24th century conventions.