Do you know if the top part of your hacked wall is also a structural beam or only those that protrudes out like the other side of the wall is considered one? Reason I asked is because I have the same idea of hacking a gap along the L-shaped wall or maybe just the side facing the hall to show case a fish tank inside the room, a tank in the wall concept. However, HDB will probably only approve a large tank if place under a structural beam.