I think nickel plating looks nicer than chrome plating and I don't know why anyone would want to dull the finish in a bathroom. These days a dull nickel finished is usually achieved with a low gloss clear finish rather than by brushing.

Radiators and/or their covers should have a plain paint finish rather than a metallic paint finish in order to maximize their radiant efficiency unless they are oversized and you want to reduce their heat output.

Lisa,
I guess it depends on the look you want. Our house is 1913 with some pretty ornate copper plated knobs and backplates. Some of our copper plated door hardware is almost like new and some of it has much of the copper worn off. We elected just to clean it up as best we could. My DH tried some metallic paint on some pieces - and just as highlights, not solid paint - it doesn't look bad, but I preferred the old worn look. So, he did spray all of them with some kind of clear finish to protect them from damp/rust and that's mostly all we did. I love that some of them look old - because, indeed, they are old.

However, if you choose to paint them, I think I would not do chrome; rather, I would do nickle. Not sure about polished versus satin, but since you already have satin on the heat register cover, maybe go with that.