Very interesting topic (and interesting device btw.)!
I did a little bit of reading. What you are looking for is ACA mode, specified in the USB Battery Charging Specification: An Accessory Charger Adapter enables charging and simultaneous data transfer for a smart phone or a tablet acting as a host.
Whether a phone supports this is mostly a software thing. There is support for ACAs in the Android source code, but it has to be enabled by the device manufacturer. So the question is whether the FP2 supports ACA mode. You could ask Fairphone support (just kidding).
Or you could just try it. You’ll need to buy an ACA compliant OTG adapter or OTG hub (those have and extra charging port). Or you could build one!
A little bit of technical background:
A regular OTG adapter shorts the ID pin of the micro USB connector to ground. This tells the phone that it should enable host mode and supply power to the USB port (to power the connected USB device).
An ACA connects the ID pin to ground via a 124k resistor. This tells the phone that it should enable host mode and go into charging mode. Power has to be supplied to the phone and the connected USB device by the ACA.
If you use a regular OTG adapter and somehow modify it so you can connect a charger, you might fry your phone.