Any ideas how I can get the PB-G3 to mount in Target Disk Mode via Firewire?
If the G3 shows the FW icon on the screen, all you have to do is connect it to one of the others with the appropriate FW cable, and it should pop up on the screen as a FWHD. If not, check Disk Utility and see if it's recognized and, if so, that it's mounted. If not, try mounting it. Never had such an issue.
See http://support.apple.com/kb/HT1661 for more info.
If the G3 shows the FW icon on the screen,
all you have to do is connect it to one of the others with the appropriate FW cable, and it should pop up on the screen as a FWHD.
and that is precisely what I reported is NOT happening. It does not mount -- not on the Mini and not on the MBP.
Disk Utility does not "see" it. Neither does Disk Warrior. And System Preferences/Firewire does not show anything connected.
The FW cable is good because when I use it to connect to a HD, that shows as expected.
Indeed, I have read the KB article. Yes, I have never had this happen. That's why I'm reporting/asking about it.
Sorry, I was mislead by your reference to System Preferences, since Network->FireWire has never been activated on my machines and all FWHD connected externals display properly. Have you tried connecting the MBP in TDM to the mini or vice versa to see if that'll work? If no one else pops in, then it's time to call Apple support, since I'm out of ideas.