Hundreds of years of acoustic instruments with rosewood and ebony boards has shown they don't need to be coated with anything. They are dark, so hide any dirt. Maple being a light-coloured wood will show the dirt, so they were covered in lacquer. I believe that Fender were the first people to use maple fretboards, primarily to lower costs (maple certainly cost a lot less than the Brazilian rosewood generally used at the time). The maple still marked fairly easily and didn't look great on B&W TV, so Fender were under a lot of pressure from users to switch to rosewood, which they did after a few years.

So there's nothing to stop you lacquering over a rosewood board, but there's no other reason to do it apart from personal taste. With the need to grain fill rosewood in order to get a smooth playing surface if it's lacquered, and the extra time taken to polish the board lacquer adds a lot of extra cost to a production guitar, which is probably the main reason why so few companies do it. Even 30 minutes extra work (especially at Western labour prices) can put a lot of money on the retail price once all the mark-ups are put on.