I need the two circuits isolated.
Why? It would be far easier to just tie the common wires together and send the digital PWM straight to the controller. I have done galvanic isolation with a high speed opto but it was because the speed controller wanted a potentiometer input and I was using PWM and I needed to convert to 24 VDC.
It is not a 'given' that you need isolation of the common return wire.
There are three wires coming out of the motor control (BEC line) White (signal), Red (5.5v+) and Black (5.5v-). As I see it, the signal (white wire) is an input. Then there is the issue that you brought up regarding feedback. Since this situation uses PWM is the feedback issue necessary?
Whether you need feedback depends on your goals. The feedback mentioned was to prevent a stalled motor burnout. So will a fuse... However, if your application includes PID control for speed regulation then, sure, you need some kind of feedback. Hall effect sensor, disk with slots and opto sensor, DC generator, or any of a number of schemes.
Also, would grounding the signal ground (isolated 5v circuit) through the opto going to work or cause some other issues?
That isn't how the opto works. On the isolated side, all you have is a transistor. It is up to you to add a collector resistor from the local power source and connect the emitter to the ground of the local source. There is no direct connection between the power supplies or across the opto.
https://www.sparkfun.com/products/9118Google has a lot of hits to 'opto isolator for motor control'
I will try and send some schematic details; These will be snippits of DWG files. Please do not be too harsh regarding this schematic, It is a work in progress.
Which will limit their utility to just those who can read the files. Better to print them off as a .pdf (using Microsoft Print To PDF or equivalent) and post them in that format.
The .png are probably ok as well.