I read this article by Coach Sommer describing how a body builder could use weights to progressively train for the Iron Cross:
http://www.t-nation.com/free_online_article/sports_body_training_performance/the_iron_cross_for_bodybuilders
"As for their effectiveness, I've seen many gymnasts capable of planche push-ups do double bodyweight bench presses on their first attempts. Conversely, I've never seen a weightlifter capable of doing a double bodyweight bench press even come close to a planche push-up initially." Coach Sommers
It's been repeatedly said that a person that can do a double body weight bench press will be unable to do planche push ups.
Instead of doing a regular "chest" bench press, what if you did a "reverse grip hip/waist" bench press. This exercise, when inverted, mimics a pseudo planche push up. By increasing the weight on each side, one could methodically work up to a full body "reverse grip hip/waist" bench press. I would suggest starting on a Smith Bench Press Machine to help counter the instability and eventually work up to a barbell and even on dumbbells.
Of course that will only help develop bent arm strength. To develop the straight arm strength that person must also hold a static hold at the top of each repetition for about 3+ seconds.
Now for strengthening the back, I would suggest doing the weighted "Reverse Leg Lifts" entitled in the Lower Back section of Coach Sommer's book Building the Gymnastics Body. A static hold of about 3+ seconds would also be necessary at the peak of each repetition. I'm just throwing out a number here, so I'll just say that a Reverse Leg Lift with half the person's body weight is necessary. It might be more, or it might be less.
Maybe even you could even work up to a support to planche on the rings using a barbell/dumbells. When you can do a full body reverse grip hip bench press, start with the barbell below your hips. Keeping your arms straight, attempt to lift the bar until it is above your chest. Keeping your arms straight, slowly move your arms down until it is below your hips again. That constitutes one repetition.
This is just pure speculation based on that article. Of course, I still believe that the progressions (frog, tucked, straddle, half lay, full) are necessary. These are just extra exercises to supplement/accelerate your progress.