Good question!
I just received the Assistant update on my phone this morning, I've been looking around. The command supposed to be:
Intent launchIntent = new Intent(Intent.ACTION_VIEW);
launchIntent.setPackage("com.google.android.googlequicksearchbox");
launchIntent.setClassName("com.google.android.googlequicksearchbox", "com.google.android.apps.gsa.staticplugins.opa.OpaActivity");
startActivity(launchIntent);
But it throws a Permission Denial: starting Intent { act=android.intent.action.VIEW pkg=com.google.android.googlequicksearchbox...
exception. So at this point it's not possible (at least not without root).
I am still looking though, but it seems Google doesn't want us to do this. Besides I am afraid Assistant would not route bluetooth audio anyway.