hi, I have worked on a project recently based on this very idea of using gravity as a potential source of energy during an collage competition. i recognize the problem u r facing and i think i can solve it. Though i have not tested the solution but i think it will work.
This kind of mechanism will require a gear train that wud reduce the final rmp i.e. a gear ratio of less than 1. then connect the output to a high torque dynamo. Now all this might seem contrary to general sense but this is a possible solution.
this was my project .