FWIW, it's possible to separate /usr on a udev system.
(IIRC, there is a warning added that will simply complain about it.)

The downside is that you'll have to use the /sbin/preinit kludge to set up the mountpoints before you run /sbin/init.
(This is the method a certain Lennart suggested on his original article about systemd.)

Whether such a kludge would be supported by the Maemo developer community is up for (heated) debate.

IMO, stuff / and everything else except for MyDocs into mmcblk0p2, and disregard the OneNAND space. It's tiny, even for a base Debian install. (or turn it into a big RescueOS, that's another idea)