Update:

> The current state seems to be summarized as follows: [...]

The current state (as of March 2017) of getting MorphOS to run inside QEMU's G4 Mac emulation ("mac99p"), which is not to be confused with the effort by the same author of emulating the Sam460 in QEMU for the purpose of running MorphOS and other operating systems (see previous comment), seems to be as follows:

"the USB controller is found [...]. [...] MorphOS now has input but still lacks some way to output video (this still needs some work)."
http://lists.gnu.org/archive/html/qemu-ppc/2016-11/msg00276.html

"this fixes mouse under MorphOS as well."
http://lists.gnu.org/archive/html/qemu-ppc/2017-03/msg00189.html