>This is probably a silly idea but how about a small coil that picks up
stray rf radiation, rectify it >and pump up a low leakage electrolytic, this
could be used in conjunction with the battery,

Probably work in an urban environment, but the specific project was going to
be out in the remote bush. This would cause your scheme to fail in two
ways: -

1. Remoteness - likelihood of strong enough RF to get charge into batteries
is not high.

2. All that green bush around will absorb RF.

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