Hi!
Yes, this kind of arrangement is possible. I have used GP3 as an input for several circuits, the R-dimmer for instance. But note: better connect a protecting diode from GP3 to +U (power). Without it the device might spuriously reset if your input voltage somehow goes little higher than +U .
Of course you loose reset input but if you configure the device with power-up timer ON, MCLR OFF, it works fine.