My neighbours have used the vent on the roof that originally vented the vent (stench) pipe from the toilet after they had removed that pipe and replaced it with an air inlet valve.
I'm thinking of something similar, but I'm planning a small mechanical heat recovery ventilation system at the moment to extract air from the bathroom and kitchen and supply it to the bedroom and living room. The heat recovery means that most of the heat from the extracted air is put back into the cold air coming in so you don't lose your expensive heating nearly as much through ventilation. Also, by lowering the humidity in the house in general it will combat the risk of any condensation and corrosion of the steel frame.
The plan at the moment is to mount the unit in the loft to extract/intake at roof level and then have inlet vents in the bathroom and kitchen ceilings and outlet vents in the front bedroom and living room ceilings. I would put the ducts from the loft to the the living room through the built-in cupboards in between the two main bedrooms and the one to the kitchen in the bathroom wall behind the bath when I reboard it (water leaking has soaked the plasterboard). This will involve making the wall very slightly thicker to accommodate 204 x 60mm flat PVC duct.
That's the plan anyway!