How to control switch mode power supply feedback voltage through optocoupler?
"Using the Isolated 282 V Voltage Regulator on page 29 LT3751, wouldn't this result in the voltage on the LT3751 switching between 0 V and Vcc?"

No, the optocoupler does not act as a switch.  The LT4430 compares the feedback to a reference, and drives the optocoupler to whatever photocurrent it needs to maintain the feedback = reference.  

So it will regulate the output voltage to the desired setpoint.  The linearity of the optocoupler is only important to the extent that it affects the loop gain of the control loop.  If the control loop is properly designed it's not a factor at all.

This is similar (but more expensive) than the optocoupler/TL431 scheme that has been used in hundreds of millions of AC adapters for many years: