I was thinking about the theoretical possibility of doing this and I found an image of how to make an OTG Y-Cable. You need a resistor (100k) to the Sync pin, and that's it.
Therefore, what the cover circuit would need is: a small Wireless Charger receiver wired to the Y-Cable's +5v and Ground and a MicroSD reader without the USB plug, connected directly to the wires. Additionally, I would recommend to add a small hub to support, together with the MicroSD reader, two other MicroUSB ports on the cover: one for charger-only, and another to have an useable MicroUSB port, as the one on the phone would be occupied. It would have to be at least 1.5mm thick to fit the microUSB female ports, which would be the biggest parts.
Well, in theory, it is possible. But I don't know if the Moto G has a big enough market share that would make any company produce something like that... Maybe I'll try to do one myself someday, just for the fun factor. ^^