Sorry I’m late to the party. I am indeed a HW user, but, truth be told, 99% of the time, I use the “silent” template in Dorico, and then I go to my organ console and play in my parts the real way whenever I make recordings.
I have been able to point Dorico to HW for a few simple things, although this is typically for something very simple that doesn’t involve much by way of registration changes and whatnot. Any time I’ve wanted to do any triggering, I’ve just hit record in HW, had dorico start playing the notes, and then triggered piston changes and whatnot manually myself.
One thing you have to have is the Advanced version of Hauptwerk to do this at all, since the HW/VST link is only accessible from the advanced version.
As @gdball mentioned above, you can assign channels for playback if you know what HW requires. And as Joel mentioned, one does not run HW from directly within Dorico; one runs it in parallel and lets HW be HW. The “VST link” is essentially a HW in-built equivalent to virtual midi patch cables.
I only have the basic version of HW at work, and my advanced setup was totally torn apart a few weeks ago since we just moved from another state, so it will be a while before I can circle back to this to remember what I did last time.
In the meantime, here’s another thread on this topic which has some helpful information: