[Lion]

Erst das DMG aus dem AppStore mounten, dann


$ hdiutil makehybrid -iso -hfs Install\ OS\ X\ Mountain\ Lion.app/Contents/SharedSupport/InstallESD.dmg -o OSX-10.8.iso


Quelle
https://j4zzcat.wordpress.com/2012/08/24/create-a-bootable-iso-image-for-mountain-lion/

[Sierra +] alternativ:
Simple bash script to create a Bootable ISO from macOS Sierra Install Image from Mac App Store
https://gist.github.com/julianxhokaxhiu/6ed6853f3223d0dd5fdffc4799b3a877