Actually, what I am now thinking of doing is instead of repartitioning mmcblk to use partition 4 as the Nemo partition, I am trying to install the Nemo filesystem on /home/nemo on partition 1 and chroot to that on startup. (as suggested by Sazpaimon)

That way there is no need to resize partitions, which makes it a lot friendly to install on a freshly flashed device.
I am trying to create the correct bootscripts for it now.