If you normalize that voltage graph in reference to ambient temperature, you will likely notice several important aspects that are not apparent.
One is that the Temperature is having much more of an impact on the Cell Voltage than you’d think.
Secondly, the is not much energy is being stored in the Li-Po when attempting to charge below 0C.
I performed several rounds of tests for a Client (paid work so I cant publish the results here). I was surprised at the outcome.
You can measure the voltage of a Resting Li-Po throughout your operating temperature range as a baseline. After the temperature correction for your re-charging graph, you should notice that adhering to the Industry (and manufacturer) Standards for Li-Po charging doesn’t really hurt the performance.
I can’t see any good reason to not play it safe with Li-Po’s.