how about something like a Asus Zenphone 2 as a starting point as IIRC it uses an atom processor. For UI, what about following the Jolla libhybris route to get wayland and then making uses of libmutter. Solus/Budgie are using it for their desktop to provide an alternative to gnome-shell. The code required from to get a custom libmutter WM running is quite small. I took a look a while back as an alternative to hildon-desktop/libmatchbox2 combination for GTK3.