Practical solution: clone your OS to SD card. Use the internal storage as a data partition.

Concept you are describing: http://en.wikipedia.org/wiki/Logical_volume_management

Additionally you can probably have /usr or /home or /opt on a separate partition and symlinked to your / partition. However the trick with this is the mess of FAT32 + EXT3 meshed together in Maemo and FAT doesn't support symlinks among other problems.