I have a locked Imac G4 also. This machine went online with an ethernet cable. I installed an Airport card, but made a mistake setting it up. Now it won't go online with the ethernet and it won't let me uninstall the Airport card, as I have no Administration rights. I tried the CMD(the key with the apple) + S solution but it won't boot into the Single user mode. I think the keyboard is suspect.
I did manage to run CleanMeUp if that's what it's called.