At least 15 years ago I had an idea for developing an iron cross and a cross pullout. I've never actually tried this, however I'm pretty sure it will work, and it would be a really fun way to develop tremendous ring strength.
Here's the idea.
Install a set of rings a few inches above the water level of a swimming pool. It would be relatively easy to perform a "muscle up" and hold a support position. From there a person could quite easily lower into a cross position because of the amount of weight a person loses when they are in the water. It should be relatively easy to perform an Iron Cross because a person's bodyweight would be a great deal less than their actual weight. As they pullout back into a support position their weight actually increases which therefore develops the correct muscles for the skill. Over time as one develops the strength all they have to do is raise the level of the rings a few inches at a time until their feet are just barely touching the water from a support position.
Again, it's just a crazy idea and I've always wanted to try it. I just don't have a swimming pool.
I guess this would also be a wonderful way to develop One Arm chins with an adjustable pull up bar above a swimming pool also.