Alright, I got a bit productive yesterday and I finally finished-up the duct work in the living room.
I had to put quite a few bends in the duct work in order to be able to use the existing (original) location. Because of this, the air flow to this particular vent isn't the best, but the living room has 2 vents (this one and the wall one), and I hardly use the room, so it's not a big deal.
Here's the new/modified run. It curves up from the main plenum (directly behind the large cold air plenum visible in the shot), up over the cold air, back down to pass under the trunk beam, and back up to the duct, so it kind of makes a twisted "W" shape.
I got this all done yesterday, but I only taped all the seams with the foil tape this afternoon (after I had already taken the photos this morning).
I originally wanted to make a full tin box, like I did for both cold air returns in the master bedroom, but since I didn't do that in the kitchen, and they didn't originally do it for the cold air in the main hallway, etc, I decided to just make a plain wood/tin combo box like in the dining room. I used antique (scrap) pine and spare tin. I made the box the same height as the dining room one, which turned out to be a nice size to clear around the main gas line.
I was very lucky to come across another piece of old barn wood that was 1 1/8" thick to fix this small 4x10 floor hole. The hardwood is the main support, since I cut it to overlap the sub-floor on both sides, but I added the extra wood for a strong and durable repair.
Patched living room floor: