I've done a prototype using a 390 LiPoly charger, with a solar panel and a battery and an XBee module as a load. The XBee is also capable to power a moisture sensor and read analog values from the sensor. I send this data to an Arduino UNO with an XBee and I store this data into an SQLite database.
It works, few hours building the circuit and some days programming the arduino and the server interface.
I want to send you a request for a new component based on your fabulous 390 charger: why don't you extend this component allowing to insert a voltage regulator ( XBee works with 3,3V ), XBee sockets and finally few holes to put some other components like sensors ? The idea is to provide a complete circuit ready for XBee, just put the XBee module and your sensors and that's it, and very versatile for power requirements: solar, USB, DC adapter. Probably this circuit could bee too much specific, but I've seen that a lot of people is using standalone XBees for similar projects like mine and a platform like this would be perfect.
Just an idea...