The problem with dreamcast pads is for some reason, whoever built them decided that the ground of the dreamcast pad would draw around 5v+ from every other source it touches. Example would be if you hooked a PSX and DC pad to a set of joystick buttons, the buttons would work fine on the dreamcast, but would only work on the PSX when you turned on both systems, because the dreamcast is drawing power, and needs that power from the dreamcast port to work.
So there is a method around this, and that's using a toggle switch to switch between grounds, the positive sides don't draw power, but the grounds are what's your problem. So just make a switch underneath your CP that switches between the I-Pac, and the DC, if you need some help with this one, just email me or something, because those switches are real easy to hook up.
What I did for my panel, was I had the I-pac wired in sync with 2 psx pads, and I used two psx to dreamcast pads to play dreamcast, so I would just unplug the psx controllers from the convertors, and the I-pac would work fine without any problems. In the future I'm going to install a dpdt switch underneath my cab, but that's when I can stop playing it, which will probably be never, but if you want to see some images of what I did with the controls, go over to my site,
www.geocities.com/cthulhuluke/Cabinet/updatepics.html and look at the "Motherbrain" which is the wiring board, and look at the dreamcast with the two psx to dc convertors.
-Good luck
-CthulhuLuke