The 12" mother board will not be able to use the LCD from the 15". If the 15 works I would keep it as is and go for the case mod in your first post. Use the ATX power supply for the HD and optical drives as normal and just mount the little dc jack from the 12" PB on the back of the PC case, and use the original laptop adaptor to power the mother board. You could use a firewire bridge to hook up drives or extend the power books own ata bus with some adaptors. Even with the power issue it wont be hard.
PB G4 15" 1.2ghz - Pismo G3 400 mhz - iBook G3 500 "Jukebox"
"SawSmurf" G4 500 - Quicksilver G4 Dual 1ghz modding...