well,
for multi systems,you can hack pads for each system then use a swtich box(printer switch box).you get the 4 port switch box and you can have upto 4 systems.you only need to switch just the ground and you can solder all the normal connection together(all the regular contacts from each interface).
this should be easy but
EASIETS would be to hack a cheap psx pad then get converter for dreamcast,usb-pc,x-box,etc.
and the question is,do you really wanna hack all the controls?

x-box controls seems great pain and the system pretty much suck for the time being.

i can explain more if things are not clear yet.
