MDLarson,
There's nothing stopping you from pulling a drive out of an external enclosure and hooking it up through another method. You'll probably find that most USB/Firewire external CD burners are simply IDE Burners (as in a standard internal CD burner) put inside a case with an IDE-USB or IDE-Firewire bridging circuit to handle the connection. As such, I'm sure that you could pull apart your Sony CD Burner to extract the drive, & swap it with the internal IDE DVD player inside your G4.
The only catch with this process is that although you're changing how the CD burner is connected, you're not changing what sort of burner it is, and thus what sort of drivers are available for it. If your burner isn't supported by OS X in the USB enclosure, it may not be supported as an internal drive either, although your chances might be somewhat higher as an internal drive (I don't think USB burner support in OS X is as great as Firewire or IDE burner support). If the case of your USB burner is easily taken apart, I'd certainly give it a go.
On the same lines you could put the DVD player back into the USB case and it should work correctly, although I doubt USB has the data capacity to handle the playback of DVD Video, making the external DVD player only useful as an external CD player.
Hope this helps,
James