[...]The logical way to avoid having to repair Btree errors is to prevent them from happening[...]
That's pretty much what mounting the OS 9 partition read-only from the start does.
My OS 9 volume no longer has a .fseventd or or .com.apple.timemachine.supported or any hidden file that OS X normally creates, which supports the fact it never gets to write to it.
Well, given both norton and disk first aid no longer complain I'm happy with the results.
That works as long as you never have to alter, remove from or add to your OS9 System folder and such…ever.
If you use OS 9 for music production or other real work, as I do, Extensions, Prefs, Help files, and a thousand other things are updated and changed all the time.
It's easy to prevent a ".com.apple.timemachine.supported" file from being created by simply locking TM and Spotlight out of the OS 9 partition in OSX TM and Spotlight Prefs.
It's not the presence an OSX-related small file or flag that causes the trouble… it's the constant updating and re-writing of a "foreign" index. Both Spotlight and TM do that. They store their indexes
on the volumes. Eventually those indexes bump into and fragment around the OS9 directory files and then OS9 Disk repair tries to "fix" it. That works…ONCE. The first time you boot back into OSX, it see the "repairs" made by OS9 as corruption, re-fixes it and the battle is on.
I have both the .fseventd and .com.apple.timemachine files on my OS9 volumes but they just sit there cause no problem as long as they're not getting updated every time I run OSX.