@tiffers
A better solution (if it worked) would be to have two partitions: one HFS to contain the Moana files, and one for OS4 in whichever filesystem OS4 uses. Then using Leo24's method you could get OF to automatically boot Moana from the HFS partition and then Kickstart 4.0 will boot OS4 from the Amiga partition. Though I have no idea whether that would work. As you say the alternative would either be to have the Moana CD in the drive when you boot and then eject it (if you can) or to have a separate external drive for CD/DVD usage and use the internal drive as a boot device with the Moana CD inserted permanently.
I wonder whether you could also add a second hard disk for Moana? So use one for Moana for OF to boot from and one for OS4 to run from? If you used a CF card you could probably fit it inside the Mac Mini case.
All of that, of course, relies on the installer actually working and Moana being able to boot from the hard drive.