Holding down the mouse button while turning on the power to the computer should cause the optical drive tray to open after a few seconds.
If that doesn't work, it's possible to manually pull open the door on the front covering the optical drive; from there I think you can get to the eject button to open it manually, though I don't have one in front of me to check. You could also probably use a straightened paperclip to open the tray and load a disc that way (it will pull back in if you turn the computer off and back on).
I'm a bit surprised, though, that holding down option doesn't work; I thought that the G4 iMacs were new enough to support the boot selector screen.