Have been thinking about this one for some time. I would it to not take over all my free time, so trying to take shortcuts and not design all the PCBs from scratch.
The current idea is to use a SOPINE A64 module as the brains for the subnotebook. It has 2GB of RAM and uses the Allwinner A64 SoC (Quad-Core Cortex-A53 ARM CPU, and a Mali400 MP2 GPU), it should be enough for this tiny thing. The module is also small enough (31mm tall) that it should fit together with the SODIMM DDR3 connector on the 54mm board. Lastly, it has a on-module SD-Card socket so that's another thing to not have to worry about.
For the screen I want to use the WaveShare 7" Pi shield, since the built-in display is also 7", the dimensions seems to be really close, but it offers a higher resolution (1024x600).
Still need a main board with charging, fuel gauge, USB, keyboard and touchpad drivers.
That's it for this short update, stay tuned for more ;)