I really thought a vent cover with a good seal that opened and closed based on the humidity would be about €30 a pop. It seems a very simple thing!
Personally, I'm looking to replace the crap "hit and miss" vent covers we have that do nothing to block draughts even if they're closed with some that will only open as and when required (i.e. when humidity levels are high in the room).
Also, they are not designed to be passive vents, they are intended to be used with continuously running extracts (at variable speeds) in high humidity rooms. These fans reduce the air pressure inside the house so air is sucked in through the vent. This is supposed to be achieved by the centrally ducted aereco extractors but we have just gone for humidity controlled stand alone extractors in those rooms.