Credits:

Invis

Sir_Wazer

How To

The script for this behavior can be found at 143B853 (To view click on the F7 byte at the noted address, open the "Code" tool on the left with the "Script" code type)

Create your Rock Climb tile and set the behavior to Unused_A5

NOTE: This works like any other HM and you will need a Pokemon that knows HM08 Rock Climb to use it in the overworld.

Happy Hacking!