If she doesn't make skin to skin contact, could you try underwear rather than a nappy. It's not really toilet training weather but perhaps the nappy rubs and sets off a sensation which leads to the behaviour. Therefore if there was no nappy the behaviour would not be triggered as often??? I may be completely off the mark.
Perhaps lots of structured activities too, so you can still get things done but redirect her to a more desired behaviour each time she begins masturbating. This way you wouldn't need to even talk about going to her room, she will just slowly realise that the only time she doesn't get disrupted and redirected is if she is in private. It will be a long process and these things tend to get worse before they get better when you start to change a behaviour but with perseverance it usually works. xxx