This reverse engineering is seeming more BS to me. 'Decoding' the seal doesn't automatically grant you the ability to use the jutsu in my opinion let alone if you could decode it. What info are you going on that allows you to pick apart the seal? Each Hiraishin seal is unique to the user. Of course there would be similarities for the teleport part, but all else would differ as said. As far as I know the secrets are kept to the users and no one has rp'ed giving the general knowledge of breaking down the seal.
And for those 'seal' masters out there who broke it down. I once again ask what info are you going off of? I don't imagine there's a book you all use to decode it.
Tl;Dr: The answer is literally in the technique, or in the case of hiraishin, in the technique formula.
Well, although it is never really explained in the series, Minato got hiraishin no jutsu from somewhere. The 2nd Hokage created the technique, and Minato was definitely not old enough to have learned directly from him, so unless there were a lineage of hiraishin users that never got introduced, Minato either learned it from a scroll or reverse engineered it.
Hiraishin is the same jutsu almost no matter who uses it. The differences between the technique formulas would either be to different dimenional voids (in line with the different pocket dimenison of Kamui line of thinking) or simply because, like writing a piece of code, each iteration of hiraishin follows a basic formula, but structures it slightly differently.
When decoding/decrypting a technique formula of any type, be it fuinjutsu or otherwise, the key is that the derived technique is either the original technique (including original formula) or a modified version based off of the original technique. Techniques similar to it can be invented without the original code, but that's not reverse engineering, that's just coming up with a different way to do the same thing without having a sample to work with, which would likely take longer than just borrowing someone else's code.
A space-time ninjutsu has to have a few basic principles in the narutoveres to make it function:
1) How does it teleport the user from place to place?
2) How does it determine the area of effect?
3) How fast does it move the user, and does this have any detrimental effects on the user?
4) What is the mode of inscription (seals) if any for the technique?
5) Is the technique a two-way streak, in that one can go to and from the different dimension that may or may not serve as a medium between teleportation.
In a technique formula where all the user has to do is think (literally, at will people put forth) most of this information has to be found in the technique formula, or otherwise has to be formed by the user manually.
Take these two examples:
http://naruto.wikia.com/wiki/Flying_Thunder_Formation_Techniquehttp://naruto.wikia.com/wiki/Flying_Thunder_God_TechniqueThe former demonstrates the use of handseals and a specific formation in combination with the marker on Tsunade. The hiraishin mark on her serves as the desstination, while the formation serves as the area of effect (more likely than not, Tsunade could also be brought to the platoon via this jutsu, but I'm not sure).
The latter is literally think and it is done.
In the case of Kamui, all of this information would be stored in the eye. Hence, "decoding" Kamui is next to impossible unless you start becoming a biological genius who can decrypt cell information as if it were the written word, and even then, you would need a living sample in order to observe its function from start to finish for the most accurate record keeping purposes.
For those who do not use hiraishin directly to decode, it is a matter of speculation. The summoning jutsu, if it were condenesd into a technique formula (as my Jounin Exams fight presented) can be a knock-off hiraishin that, with some tweaks, could rival it presuming knowledge of will-activated technique formulas is known to the user.
Shadow Migration and Flying Shadow, two of my custom techniques, all started with the Shadow hell technique. By finding a way to go to and from the shadow dimension, the gate of shadows turned into shadow migration. By directly connecting the shadow gates and using the shadow dimension only as a temporary medium or void, Flying Shadow was born, though to keep it from being literally shadow imitation hiraishin, a handseal is required in order to perform it, since unlike hiraishin I would normally have the option of just staying in the alternate dimension.
At will I can use shadow migration in conjunction with shadow seals, but using flying shadow requires a handseal, ICwise, because not only must the user be sucked into the shadow, but they must also appear at their destination considerably faster than they normally would, adding a degree of complexity to the technique than was already present.
I hope that cleared some things up for you Shadow, or at least jogged your brain.