The door operates on a simple spring and latch combo. When the tray comes out, it pushes a little button, which drops the door. When it comes back in, he button is released, and the door comes back up. I haven't looked too closely yet, but I would guess such a part would easily be accesable after removing the optical drive.
I opted for the combo drive to save a few bucks, so the brand in my G5 is a Toshiba. The speeds of this drive are as follows: DVD-R Read 4.8x, DVD-ROM 12x or 8x (depending on dvd type), CD Read 32x, CD-R write 32x, CD-RW write 10x.