Seems like the combination of PowerPC Mac, disk encryption, and Linux (at least in Chimera) is a rather temperametal one...
Ended up having to add GPU and USB HID modules to initrd preload list (in Chimera, in /etc/initramfs-tools/modules):
nouveau
radeon
usbhid
Otherwise I'll get into a situation where I could neither see the password prompt nor input any disk password :akkoderp: