LiFePO4 in cold: charging
When working on the fringes of specifications, it is always best to speak with an applications engineer at the manufacturer. They often have access to data that is not part of their published specifications.

With that being said, you should give careful consideration to thermal management within your design. For example, a large insulated thermal mass in contact with the cells will retain heat generated during the exothermic charging cycle. This will help prop up the ambient temperature of the cells during non-charging periods - such as at night.

The choice of enclosure color can also play a role in raising the average ambient temperature of the cells.

You may also go so far as to use your solar power to warm the cells until they reach an acceptable charging temperature. While this is not the most energy efficient approach, it is a pragmatic one that may enable the application.