Can the Powerbook's SATA controller manage two drives?
Could the Combo drive be replaced by a HDD?
This doesn't exist. Its PATA Ultra ATA/100 (ATA-6), not SATA. But to answer the question with PATA, yes, it could handle 2 drives, and yes, you could remove the combo drive and probably fit another 2.5" hdd in there -- with room to spare.
Via a thread the other day here is a link to http://www.mcetech.com where they offer an adapter to put a HD in place of the optical drive on PowerBooks. IT's called the OptiBay Hard Drive.
They have this to say about 12" G4s:
*NOTE for 12" PowerBook G4 Owners: We highly recommend that your drive be professionally installed. The installation of the MCE OptiBay Hard drive into the 12" PowerBook G4 involves the removal of some 40 plus screws and parts and is not for the faint of heart. Only an experienced technician should attempt this upgrade into the 12" PowerBook G4.