IIRC, the 6th wire is so the reset switched can be used as a menu/item select button instead of a gamecube controller.
[Never knew about this. Could you explain further?
the 6th wire is for updating
It supplies the extra voltage required to flash the atmel chip.
It can't be a requirement for DVD updating... maybe using the code that the WiiKey uses does require this. But if OpenWii can DVD update without this extra wire, and they use the same chip, than it can't be just for DVD update.
But again, it might make a difference on how it's coded, I'm not really sure.