You can repnotify anything, just repnotify only a single function at a time and it works. In your case just Set Material on the Mesh, select the material index you want to change and change it to the material you pop in as the replacement. Then toggle that on the Variable You originally created, it can be a bool that is called TurnOnNewMaterial or something.
Take out all that other stuff you added and only call the part where you set the material, nothing else. You don’t need to cast it to a character, just repnotify in the character and call it somewhere else.