Basically, on the right side of the optocoupler (=LED side) the system has essentially a voltage reference (such as TL431) monitoring the output voltage and controlling the LED current accordingly,
You could get the same effect by just using an ordinary transistor instead of optocoupler's output transistor, and regulating the base current of that transistor directly. Because an optocoupler has very low current gain (<<1), and any transistor has much higher current gain (typ.many tens to few hundreds), you have to re-design the voltage monitoring circuit accordingly. Key here is right level of gain in the feedback part for loop stability and good enough regulation. You have also consider, while doing that, the wide range of transistor gains. However, compensating well enough for that gain variation should be reasonably easy.
Good luck!
ted