With FilteringEnabled, how can a script tell a local script to do something?

I'm having some difficulty with FilteringEnabled. Especially since GUIs can only be changed with local scripts.

I have a script in the ServerScriptService. I need it to tell a LocalScript in the player's backpack to change an ImageLabel's ImageTransparency.

What I tried: I tried using a BoolValue and .Changed in the LocalScript, but it was having issues detecting values changing.

So how can I make a script tell a local script to do something?