>  /NetBSD-1.5/macppc/INSTALL.html says to use Drive Setup 1.9.1
>  or higher to format the partitions from MacOS. Is that a typo?
>  http://asu.info.apple.com only has version 1.7.3.

Drive Setup 1.8.1 did work for me. You can find a copy of it on every 
Mac OS 9 install disk.

>Does the HFS(+) bootloader partition need to be brand new,

No.

>or can
>  I just throw ofwboot.xcf onto an existing non-system partition?

Yep!

>  And will the A/UX partitions be visible when I boot in MacOS?

No!

But keep in mind, that Drive Setup initializes your whole disk. So 
back up all your data!

Chris