Hi there!
I don't think we have a guide on how to add special effects to items. I use the FOClassic engine.
My particular case is something I saw on fo3: I want bags and backpacks to add a percentage to player's max carry weight when equipped in hands.
I looked at how other things are implemented like the mirrored shades, but I'm not sure about all the steps I need to take to properly add some logic to an item. It would be great if someone could share the steps. Thanks!