If I remember correctly, the B&W G3 machines had hardware DVD decoding under OS 9, and had a DVD "daughtercard" attached to the video card that was required to view DVD video.
Mac OS X uses software decoding, so the daughtercard isn't needed for DVD playback.
As far as I know, there isn't a workaround you can use under OS 9. I do not know of any alternate DVD players for OS 9.
What kind of video card did you put in the machine?