Well it seems 8.0 has no problems. It boots, shows strange symbols too but screen switch black and then normal console shows up.
FYI I started 8.0 this way :
qemu-system-ppc -boot d -M mac99,via=pmu -m 1024 -hda /srv/disk-images/mxqc1-test.img -cdrom /mnt/NetBSD-8.0-macppc.iso -net nic,model=e1000,macaddr=52:54:00:12:34:fa -net bridge,br=br0 -prom-env 'auto-boot?=true' -prom-env 'boot-device=cd:,ofwboot.xcf netbsd.macppc' -vga std &
And 8.1 this way :
qemu-system-ppc -boot d -M mac99,via=pmu -m 1024 -hda /srv/disk-images/mxqc1-test.img -cdrom /mnt/NetBSD-8.1-macppc.iso -net nic,model=e1000,macaddr=52:54:00:12:34:fa -net bridge,br=br0 -prom-env 'auto-boot?=true' -prom-env 'boot-device=cd:,ofwboot.xcf netbsd.macppc' -vga std &
As you can see, except from the iso there no difference.
Forgot to mention on last mail but both iso generate this error while booting :
font-adr: interpret: exception -13 caught
call-method 'usb-kdb-ihandles': exception -21
Possibly due to "via=pmu" but I don't think it related to the "strange symbols issue".