How to measure Li-Po battery leakage?
The discharge rate dependes of several factors and will vary from manufacturer to manufacturer.

A good Li-Poly typically has 2-5% self-discharge rate per month. If it has a PCM (protection circuit module), that module will draw some current, expect less than 1uA for such a tiny cell. Parts of lesser quality may draw 30uA or more.

Self-discharge increases with temperature. I believe it also varies with cell construction (cylinder, prismatic etc.)
Quality varies a lot from manufacturer to manufacturer so you have to ask for the exact specs.

UPDATE

I don't have much experience working with the current levels you mention. Wouldn't the ground current in the charger consume most of the 10-20uA? There is a lot of interest in energy harvesting and someone else can probably give a better answer.
Slow-charging the battery is OK provided you cut off at 4.2V. My guess is that 10-20uA is not enough to offset self-discharge and other losses, but I'm frequently wrong.