Light weight power generation

Hello,
My purpose in writing this post is that I am looking for ideas to help with the design of a project that I am interested in working on. Rather than ask a specific question then, I will just overview the problem, and describe what I am considering for a solution.

I work with a research group that flies high altitude balloons. The longer the flight, the more batteries need to be included to power the system during flight. Thus, I was somewhat interested in looking at energy sources other than batteries. One major possibility is solar power. While this type seems more promising, I have decided to look into another source - namely extracting energy from balloon/payload oscillations.

The balloon/payload oscillations provide motion on a single axis that can be transferred to rotational motion by pushing a wheel against the main line. This can be turned into electricity by driving a DC motor with the oscillations (I'm pretty sure the output will be less than 200mA at 5V - however, who knows, it could be less than 1mA.) The output will be AC, because the motor will be turning back and forth. Thus, I will need a AC/DC power board to convert the voltage to DC. Then I can just store the charge in a capacitor to account for the changes in the magnitude of the oscillations - so that a constant current can be drawn.

Some problems I see:
I don't know how efficient DC motors are, or if there is a better way.
I don't know how heavy a AC/DC power board will be
I'm not sure how changes in the magnitude of the oscillations will effect the system

Any suggestions?
Thanks!
-cjkogan111

I have some experience with high-altitude ballons. I think, extracting energy from the payload oscillation is a dead end. At altitude, there will not enough oscillation. Simply, there is not enough atmosphere to blow the payload back and forth. There might be some inclinometer data that says otherwise.