Seems feasable. I dont have a ES-8, as a disclaimer.

You can assign “output Volume” to any CC in th MIDI CONTROL menu in H9 control.

Then, if your controller can send the right value… it can set the output volume, just as you say. A little fiddling to decide what cc value corresponds to the right volume, and you are there.

Some people in the past have had hiccups sending a PC-then-CC right away, without a little delay (for the patch to load maybe?). So if you run into trouble, that’s probably it.