GameCube disks don't spin backwards. There's a very easy way to check that. Start a game, turn off the power, and open the door. The Cube's disks spin clockwise, like every other console does.
GameCube disks read from the outside to the inside, which is unlike any other console. Take out your disk, close the door, turn the system on, then off again, open the door, and see where the laser head tried to go. A system like the PSone makes a dive for the inside, while the GameCube makes a dive for the outside. Simple.
Because of this, if you tried to "burn" a GameCube disk you'd have organize the data backwards, and then put a precision amount of blank space on at the "start" of your burning so that the start of your GameCube game ends up landing nicely at the "end" of your burn. That's do-able. But that's not enough. You'd have to mod your burner so that it spins backwards. Why? Look at
this quick drawing. Could you replicate the GameCube's pattern just by organizing the data so the Cube's "start" ends up at the normal disk's "end"? Nope. Your burner would have to spin backwards, even though the Cube doesn't. And I don't think a CDR/DVDR is even set up to handle that. You'd have to
make your own GameCube disks. Just going into Office Depot and buying a pack of mini-DVDs is not going to cut it.
I've seen a lot of people spout a lot of crap about "Oh, it's just the filesystem. I can break the GameCube's disk format. I'm gonna be famous. You just wait and see." But nothing's ever happened.
I still say that widespread piracy won't be possible on the Cube until they come out with another mod that also lets you wire up an external DVD drive/writer (heck, you may as well throw in a hard drive while you're at it), in addition to this mod chip.
Edit: Made my drawing a little bit clearer.