Umm.. this is CthulhuLuke, but IE 6.0 sux, so I'm cthulhuLuke2 as guest for now.
Anyways, yeah, I wanted to do the same thing and I've only hacked one psx pad before. It's real easy to do, it just takes time and the patience of knowing that the worse possible solder point that you think is going to work will pop off and you'll have to do it again. I hate that. Here's a diagram of what my control board looked like, this is a sketch, the final actually turned out easier if I put alllll the terminal strips on one side, I don't have any pictures of it until I get a new roll of film. I highly suggest doing the PSX and I-Pac thing, then using PSX to Dreamcast convertors for your dreamcast if you have one, because being able to play PC, PSX, and Dreamcast is the best way to be.
-good luck
-CthulhuLuke