Because Linux installs(Kubuntu based at least) take up lots of space, would it be possible to mount a MicroSD card to /usr AND /home and install apps there, saving internal storage space for the OS? And because MicroSD cards come in sizes up to 1TB(http://www.amazon.com/Samsung-micro-car ... icrosd+1tb) then allowing them to be mounted on /usr AND /home, could give desktop class storage to phones running Plasma Mobile.

I think if you would use a btrfs filesystem you can just add the storage to the internal to extend free space.
This works quite nice on SailfishOS. Though it might be a risk aswell as btrfs tends to behave strange when running full.

Maybe AUFS/UnionFS can be used in that cases as alternative.