Another reason for the reuse of solarpanel+battery -charger devices is that they're designed to charge their internal batteries from low current source, and have built in booster for boosting the battery voltage to 5V..

It's probably not entirely reliable to get N900 to charge from 200-300mA with bme, or atleast I've found it a bit unreliable with the default software. So the external storage battery+panel+teg would also serve to discharge the harnesses energy at a rate N900 is happier with.

Those are some impressive figures for your teg though.. How do you maintain a 300 degree temperature differential I wonder?