One more thing:

All the files within the fiasco image - is it really enough to extract and use rootfs.jffs2 only? First I thought so, but now it seems somewhat naive. Re-checking the contents, I found rootfs.jffs2 lacks not only the /mnt/initfs/-directory present on the 770, but also a whole bunch of nodes within /dev/ (and probably much more, I just have to figure out...). No wonder this doesn't boot.

I guess what I need is a way to put together all the contents of the fiasco image in the correct way on my desktop PC - this means learning a lot about how the maemo boot process works, I'm afraid