The original Cthulhu is PS3/PC
The Cthulhu MC is multi console (PC, PS3, Xbox, PS2, PS3, Wii, Gamecube), but of the many systems it supports dreamcast is not one of them. If you look at my arcade setup this is why I have hacked dreamcast pads as a part of the setup.
http://picasaweb.google.com/masakakoi/CustomArcadeControlPanel?feat=directlinkWhat your saying is true, your board will show up as a windows gamepad if you swap out your hacked DC PCB for the Cthulhu, and if you do it right you can wire them into a dual pcb setup and have both of them work for the same control panel.
If you get the Cthulhu MC you may want to do what I did and a few others and just use one of the outputs and wire it to a RJ45 jack and then hack the console cables into RJ45 so you can use any console you want and just plug/unplug the cord you need rather than having all the console cables soldered onto the board like some kind of freakish cat o' nine tails.