This is probably not that deep a question, but how can one extract power from panels in low light? A 6V panel would dip to ~2V or so, and power output would sink lower. In conventional systems, how is power extracted (and stored) in such conditions? MPPT would only find the max power point, which would be terribly low. A battery wouldn't get charged because there isn't enough potential to drive current. But the sum total of this power throughout such a day would be quite high. So how can this power be extracted and stored?
I'd say you want an energy harvesting boost converter. These work down to very low voltages on their input (some as low as 0.25V I believe) and can provide a trickle of current for keeping a battery charged or for charging up a capacitor.
Here's one from TI that might fit the bill: -
It's got some quite useful applications. You should also try linear technology and Analog devices too - search for energy harvesting.
By my experiences, the maximum power power point of a solar cell doesn't change that much in different light conditions. For a (solar powered) project I used a cell specified with 4V maximum voltage and 200mA short circuit current. I graphed its characteristics in different conditions - starting with normal room lights, then my desk lamp in different distances up to high noon sunlight (though this was in november...).
Worst case was the normal room light with 1.75V for the MPP, up to about 3.5V for direct sunlight. For most other conditions, it ranged from 2.5 to 3.0V. Setting the MPPT to 2.5V gave maybe 10% below maximum power for the sunlight setting - the power graph is mostly flat in this range.
So if your solar cell can deliver 2V even in low-light settings, set the MPPT tracking just below that voltage, and you will get nice trickle charging. The TI part mentioned above is fine, my project used a LTC3105 from Linear. It is also designed for energy harvesting applications. Just set its output voltage right below the charge termination voltage of your battery, and you are fine.