> your boot.img has to be on a HFS+ partition

HFS actually. MorphOS lacks write support for HFS+, which is required by the installer.

> Some info https://www.morphos-team.net/guide/usb-boot

…or there: https://library.morph.zone/Dual-boot_MorphOS_and_MacOS_X_on_a_Mac_Mini_G4 ;-)