Two suggestions coming through over twitter:
A good start is to reduce your reciever's gain as far as possible.
Hopefully you can snuff out the antenna's rear lobe that way. Headphones really help there too.... (read more)
Here is another TEG that looks promising.
Texas Instruments bq25570 Ultra Low Power
Harvester Power Management IC
Yeah, open-source geolocators would be a great addition...in principle, it might not be too hard (famous...last...words...). Also, they are great for migration-level studies where location accuracy isn't essential, although I am not sure how inaccurate they are. Thanks for the link htarold. (read more)
This is all very interesting. The solution you describe seems eminently doable and in fact quite cheap. So doable and cheap that it may behoove one to also ask what the higher level vision is, in order to further constrain the solution.
For instance is there a wider... (read more)