How can I reinstall Mac OS 9 on a G3 PowerBook's original hard drive?
I tried dding the the Mac OS 9.2 Universal Installer to the hard disk, but that didn't work.

@jaume put me on the right path, so I dd-ed the ISO to a USB stick, held down option at boot, and I'm able to install Mac OS 9 to the original hard disk. Success! Thank you!

The real thing I learned is that Mac OS 9 boot volumes need half a dozen special driver partitions. Older versions of Mac OS X, up to 10.12 or something, let you partition OS 9 drives correctly using the Disk Utility which had an "Install Mac OS 9 Drivers" option. I used hdiutil info to check that the ISO had these partitions.