I have also added hard drop to NES Tetris. Heh. What are the odds?

This hack strays further away from vanilla NES Tetris though and contains further speed, control and graphics modifications; and lacks that fancy looking ghost piece.

https://www.romhacking.net/hacks/4973/

My approach of hard drop implementation was different, honestly i just looped the piece down routine until it hit the ground. His approach breaks the scoring system though, mine doesn't =p

Anyway, this was a great article! Nice rundown of the NES architecture.