I bought one of these UPS boards. It comes with a 3800mAh battery that played a MIDI loop on several Fluidsynth layers for 3.5 hours. It has various LED indications of status plus serial status updated every second with input status, output voltage & current battery capacity. It also has a GPI pin that goes low when battery is about to expire.
The main purpose is to decouple the PSU from the Zynthian, allowing current peaks to be delivered without the need for an oversized PSU. It also handles temporary loss of power but could be used for off-grid performance like busking. I am yet to test duration under load with external devices powered, e.g. USB MIDI controller. I am sceptical of its battery capacity measurement which sometimes continues to grow for some time after power is removed but the GPI seems valid.
It is pretty cheap and it might be worth adding a software module to monitor this (and other) UPS. I will knock something up.