How to use same 'input' for two circuits
Your assumptions seem correct.  However, just make sure:

1) The voltage of the power supply is the same on the input and output of the optocoupler, it could be different and could cause trouble. (I'm assuming it is based on your drawing)

2) The resistors you use, limit the current accordingly.

3) You can go for the first option, but you need to place a resistor (try 10 kilohms or higher) from the colector at the output of the optocoupler to the PIC pin input.  This will prevent your PIC becoming a load and provoking malfunctions like false triggering or not working at all, etc. of the external circuit (Again I'm assuming the voltages are within the specs of the devices, if not place another optocoupler)

4) The other option you thought, can be made but, you resistor on the optocoupler probably will have to be changed.  In that configuration, your signal would be inverted (when opto is off the external circuit will be on and vice-versa, if that's what you need then it's ok). And finally, you should also need to calculate the emitter resistor on that BJT transistor. Hence, this could be a more complicated solution.