It is a good concept Mikael, I have seen something similar done on B&B. I think if you finished the shelf with the right coating there would be no need for the tape or worry about moisture. What I like about it is if you had the templates you could make a custom one tailored to your desires. Something that takes all your favourites either in sets or by type.
Personally I don't think you need to hang the brushes at all, I would make small hollows in the wood so that each handle sat in a shallow cup. I still hang them right after shaving on my stand that is in the bathroom, but next day they get stood up on the shelf. I hang only for convenience and mostly habit, I really don't believe hanging them bristle down aids the drying process one way or another but if you like them hanging then hang them by all means.