I think you’ll have a tough time insulating such a vent so that your bathroom doesn’t end up cold to make up for your attic being less stinky. I don’t see a vent cover that is openable by air pressure being heavy enough to keep the cold from getting in when the fan isn’t on. Though it might help to make the vent push the air downwards so that the cold air would have to creep upwards to get into the bathroom.
I am out of my element for this, however, so this is more advice to look into this angle rather than advising to not do it.