progress! my mistake was directly installing OS9 onto a partition created by Tiger's Disk Utility - i needed to initialize the disk using the OS9 installer's Drive Setup. i can now boot into OS9!
No issues with using Tiger's Disk Utility for that. However, one thing that DOES cause an issue that I was able to confirm is if you try to use the "Restore" function to restore an image onto a partition, and then that fails. If that fails, the partition "looks" fine, but it might be unusable for booting into OS 9 with it (I forgot whether alt-booting still worked or not, though), even if you rewrite all the files.
Another issue I recently discovered with OS 9 bootability is that i.e. with a scenario of 3 partitions like the following:
Disk 1 - 20GB
Disk 2 - 150GB
Disk 3 - 20GB
If installed, OS 9 won't boot from Disk 3, but will boot fine from Disk 1 and 2. However, if you rearrange them in the following way:
Disk 1 - 20GB
Disk 2 - 20GB
Disk 3 - 150GB
Then OS 9 can boot from any of the 3 Disks just fine. (Again, I would have to double-check if alt-booting works regardless).
In my case, this was observed on my Mac mini G4 "64MB VRAM SuperMini" 1.5GHz model, but I suspect this is applicable to all OS 9 machines, official and unofficial. It took me years to realize this was an issue.