A pinout for the connector used by the Dreamcast is available here, but I don't really know if its going to help all that much. Because of the way that data is transmitted from the peripherals to the Dreamcast (and vice-versa), its pretty much impossible for a normal person to make a Dreamcast controller without cannibalizing a controller for the interface hardware.
The rest of Marcus' website has all kinds of fun Dreamast-related stuff, including detailed information on the wire protocol used by devices attached to the Dreamcast. Its pretty much the best source of information outside of Sega's own documents (which of course would be illegal for you to obtain).
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