I successfully pushed my luck. I now have a strawberry iMac G3, with an optical drive that will actually read discs the G5 burns for it, 512MB of RAM, a working FireWire port in both 9 and 10.2 (it detects my DV digitizer in iMovie and can record from it), and 600MHz of powah'. Cleaned it thoroughly and got all the dirt out, inked out/polished up nicks in the coloured plastic, fixed a short in the CRT, put it all back together and it's now deliciously pimped.

My thinking is that there are two connectors on the G3 card for a second FW port that was never made available, since there is clearly a second punchout on the Sonnet-provided ports panel, and by sheer dumb luck I only trashed the contacts it wasn't actually using.