Can I add an HVAC supply vent directly to the trunk?
That should be fine (I've seen it done many times in basements), with a few caveats:


You're right that there will be more flow with that setup than with a length of round duct. Add to that the smaller room you're feeding, and you can see that you'll need to dampen with an adjustable vent.
By doing so you may introduce noise. You have a shorter sound path from the furnace itself, and adding a damper vent will likely cause white noise at that location as well.