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.
It’s not about the smell, its venting moist air into the attic which causes it to get trapped and promote mold growth like with a sweaty gym sock. The people who built my house just used string to point the vent pipes toward the standard screened roof vents and even that has been an issue. They should ideally be exhausted through the roof with purpose built vents. Even exhausting through the wall or soffit can be an issue as air gets sucked in through the soffit which can pull that hot air with it.
Yup. With the vermiculite insulation in my attic and the moisture from that vent, I practically have an ideal mold farm up there right now. I need to fix that vent before it gets too much worse.