Just spoke to an interior designer for big multinationals. Mentioned that the whiteboard paint was a good idea and cheap but what eventually happened was that the colour was at some point never removed so would start to look like a shade of grey. At that point it was worth just painting over cause it is cheap.
Then told me about magnetic glass boards which are much easier to clean. But expensive.
Re: decoration. Sounds like a great idea. Would be cool to do with one Lego brick per member.