If I got my hands on a few of those it would be interesting to adapt Cosa :-) I use basically the same hardware for prototyping; Arduino Nano and the Nano IO Shield which has a NRF24L01+ and ZigBee socket. I am planning support for at least three major types of nodes; basic sensor nodes which should be scalable to ATtiny84/85, routing nodes for mesh communication and bridge nodes from NRF24L01+ and other small scale RFs to ZigBee/WiFi/Ethernet/LTE, etc. The last type of node will probably be Linux based. The others will typically be Arduino/AVR based. The Sparrow seems to be perfect for the two first types of nodes; sensor and routing nodes.
Right now I am working on improving the Cosa TWI/I2C bus handling so that it will be really easy to add Cosa/Arduino based slave units. This would allow partitioning and distribution of functionality when needed. A new level of shields such as LCD, Network, etc, would be interfaced through TWI instead of having to compile/link drivers into the sketch. Instead simple proxy libraries can be used and the code base becomes more scalable. With an AVR processor on each shield they can be used stand-alone or integrated over TWI. This was basically the foundation of the dtools research project at Stanford, http://hci.stanford.edu/research/dtools/