It's my understanding that the Particle SoM's allow/require you to provide your own circuitry for Battery Charging.
In addition to the reference design, there are several other designs (such as this) from Community members.
You could also go the route of using a 12V Panel, Battery, and step down converter without modifying your existing device/enclosure, but it seems that the space might be an issue for your application.
I'm sure you will get better feedback for the BSOM than I could provide 