Hmm...good question, although I'm afraid it might not be possible.
All the other HMs (except Fly) are activated in two different ways, either through the Pokemon Menu, or through interaction with the obstacle. Because of the nature of those HMs, they absolutely need to display some sort of message confirming the player actually intends to cut the tree down, or smash the rock, or surf.
Fly and surf are different, though. The only way to activate them is through the Pokemon menu, and because of that, there's no need to confirm the player actually wanted to do that, so I don't think there's a script associated with them.
If you want to try searching, I recommend you search for certain commands, such as "checkattack MOVE_FLASH" by converting it to hex, and searching through the ROM to see if you find anything. Good luck!