I don't know the structure in 4th and 5th generations but as far as I know, 2nd and 3rd gen can both be hacked easier. The data stucture in 1st gen is very different from the 2nd and 3rd gen in scripting part (otherwise a lot the same as 2nd gen). There is no straight pointer that would lead to the script of a person. Instead, still a byte (or two) inside the event data of a person loads some ASM code which you can modify (not even in pointer format).
2nd gen is much easier. If you understand how pointers work, you can do almost anything you want.
I think it's pretty much the same with 3rd gen games. In 2nd gen, a 2-byte pointer inside the event data, locates the script of a person but scripting can be done still "pretty easily" thanks to the research Tauwasser has done.
*Search for Tauwasser's Scripting Compendium if you want to see more.
I may have misunderstood your message - by "structure", you can mean a lot of different things.