I don't think I'd want to put something painted that way into the tank for several reasons. Fish, snails and other inverts are going to try and feed off the rock. Many have fairly powerful jaws and teeth, and would likely be eating your painted surface. This wouldn't be ideal. Also, in time, it's going to get overgrown with algae and other life, so the paint work will get hidden under that.
The other big reason is that no matter how great your artistic skills are, it's almost certainly going to look fake. Since it's quite possible to keep living corals in the tank, it becomes more a question of why have a copy when you can have the real thing?