Driving multiple Pi GPIO using Arduino GPIO
The diode circuit should work but not safe to parallel multiple loads to single drive. The voltage across the pi inputs will be diode drop plus 3.3 V (VCC of Pi). The current through the diode connected externally will be about 1.3 mA multiplied by 8 will give ~11 mA. Also, the input capacitance of all the Pi's inputs and 8 cables connected will make switching that current at high speed will get trouble with rise time and fall time too.  

For your particular application, if you can buy one 74LVC245 buffer IC, please go ahead with below solution.  

You can simply connect the Arduino single output to all the 8 inputs and derive 3.3 V level signals from the outputs. the advantage is that it is simple to use (Breadboard friendly) and easy to connect to.  

 

The input voltage can be up to 5.5 V. Hence, it can accept the Arduino's input and drive the Pi's input easily.