I'm on here trying to figure out what to do with my own piano in my new house. I noticed acm's comment about don't worry which way the lid is pointing--the sound will go everywhere. That is the truth! It is for aesthetics that you want the lid to open to the room. I personally would rather look out the window while I play piano than to look to the people in the room. About the heating register... You don't want the heat being blown on the piano--true! What my Registered Piano Technician told me was to take the register cover off and cover the bottom with plastic wrap to seal it off. As long as you only do this one one register in the house, it should not place too much wear and tear on your furnace.

If this space does not need to be a tv room, I would take the opportunity to create a music/library space. Do a wall of built in bookshelves where the accent wall is and get four super comfy club chairs in either leather or velvet, a couple of ottomans, and a couple of end tables. You could group all four together in the middle of the space, or do two groupings, one closer to the piano.

@Tootsie: The fireplace is non-working/covered, so there's no issue there. It is visually the focal point of the room, though.

Definitely leaning towards neutrals, as this is the color scheme of this entire level of the house. All walls are BM cloud white. All windows have black curtains. Flooring is the same gray/taupe hardwood throughout. I was thinking caramel brown leather to pop out on that black shiplap wall, but may be too pricy for 4 chairs. Definitely don't want anything traditional or fussy (which I KNOW goes hand in hand with the mahogany piano but alas I cannot change that lol!) Furniture in the adjacent rooms is cream/gray/wood. I'll upload a picture when I get home to give an idea