Check the Openfirmware version... I think 1.1 and up does read ext2/3 filesystems... maybe you need to update. There are other posts on that. Do a search.

If it is something else maybe it does not work... if you have a MorphOS install in the disk you can just copy that kernel file to there and boot from the amiga filesystem. To do that you can use a Gentoo LiveCD or during install.