Exactly!
So, one wonders with OS 9.1, can Bert use the C key, or the Option key? While that link is informative, it does not specifically mention anything about any version/"flavor" of the Mac OS.
I tried booting into the G5 HDD at startup. I tried two different approaches. Neither approach worked.
For the first approach, here's what happened: (1) I booted up the G3 while holding down the option key, (2) I saw a display of bootable drives, i.e., the G3 HDD and the G5 HDD, (3) I selected the G5 HDD, (4) the Mac logo appeared on the screen, and underneath it a little swirling wheel. The logo stayed onscreen and the wheel kept swirling. After ten minutes, the logo was still onscreen and the little wheel was still swirling. I turned off the G3. I repeated the approach, with the same results.
For the second approach, here's what happened: (1) I booted up the G3 while holding down the T key, (2) the Firewire icon appeared onscreen. According to the article <
http://osxdaily.com/2010/04/07/how-to-boot-a-mac-in-target-disk-mode/>, this appearance of the Firewire icon onscreen "signifies that target disk mode is detected and working." The article goes on to assert that "in a few moments, the Mac will boot as usual and the target Mac’s hard drive should appear on the host Mac’s desktop, allowing you to access it like any other external drive." This did not happen. After five minutes, the Firewire icon was still onscreen. I turned off the G3. I repeated the approach, with the same results.