It looks like it might have some potential. A lot of its utility depends on the details of the design. Like:

How rugged is it?

Will it withstand normal abuse, being dropped, exposure to sand, mud and water?
How long will the string, bearings and return spring last?
If the string breaks or wears through can it be replaced with commonly available materials and without tools?

I'm reminded of the much larger and heavier manual generator that was used to run radios in WW2 when other power sources were unavailable. A sturdy cylindrical body with legs and a two-sided crank like you see on a bicycle. Usually it is depicted as being cranked with the arms but it could be worked with the feet. Evidently it was very ruggedly constructed as it was used by remote and hostile locations world-wide.