Another interesting idea is to use a supercapacitor. Then the motor will charge the supercap and the supercap will charge the battery via Joule thief- in order to reach the required voltage to charge the NiMH (or LiIon).
A nice point in this approach is that I can charge the capacitor during night and discharge it daytime. Or using 2 of them, each for 12 hours.
+++
I searched the internet about supercaps as chargers but no result so far.
I suppose no matter the voltage (usually 1-3 V, very variable), the cap will charge anyway. Then the problem is how to gradually discharge the supercap into the battery.