Requires RAGE Multiplayer 1.1.0 and above.
Put rappel into your server's client_packages directory, then add require('rappel'); to client_packages/index.js.
RAGE Multiplayer's 1.1 will sync the rappelling action so there isn't any need for server-side files.
Get in a helicopter with FLAG_ALLOWS_RAPPEL (Maverick, Police Maverick and Annihilator) flag as passenger and do /rappel.
* Credits to Trevor Productions on YouTube for the preview image.