RS-485 bus with PoE bus
You can certainly do this within limitations. The main thing you have to be concerned with is the voltage drop that will occur along the wires. One reason that high voltages such as 48V are used is that the drop along the cables is a smaller percentage of the total voltage than say if 9V or 12V were passed down the wire. Put another way, you can pass more power down the cable at 48V than you can at 9V or 12V when living with a given cable voltage drop. 

Now with RS485, even though it is a differential bus, the receiver parts used will have a maximum common mode voltage range specification that must be obeyed in order to ensure reliable operation without sacrificing silicon health. The voltage drop in the cables due to the power delivery scheme have a direct impact on the common mode voltage that the receiver will see with respect to its RS485-A and RS485-B differential pair pins. 

Also be aware that RS485 modes are not particularly low power when you consider that the driver must be able to drive the terminations on the bus. Typically there are required terminations of 120 ohms between the differential pair at each end of the bus. This means that the drivers are sourcing and sinking current across a net 60 ohms when the bus is active. If your driver is pushing out 5V signal levels this corresponds to a current of ~83mA which is probably 5 to 10 times more than the typical low power microcontroller node current consumption.