Wall planche push ups are a very good idea when you're able to easily do 5x5 smooth reps of XR PMP. Wall maltese push ups come when 5x5 wall planche push ups are no problem. Body must remain straight and slightly hollow the whole time.That's Coach's recommendation in the book. A static wall hold isn't bad, but you are running a big risk of overusing the biceps tendon at this point. Just be careful not to do too much! It will definitely help, but too much too soon will set you back many months.
Keep in mind those come in just behind unsupported planche push ups in terms of difficulty. Done correctly these will be very, very, very hard. Don't let your upper arm squeeze in on your body.