I finally was able to try SwitchResX on my G4 after replacing the hard disk and installing Tiger. It came close, but no cigar. It will also do 1360x768. When adding a custom resolution, entering 1366 changed to 1360 after moving to another field.

I also tried DisplayResX. It attempts to determine the resolutions by itself, and won't let you add any extras without paying for the program.

The only possible cause I can see at the moment is that both the G4 and the PC have ATI video cards. I do have another PC with an old Vanta card in it. I'll try that out later today.