Ok, so I'd like to create weapon mods for guns that don't have any (most specifically, a scope for the MultiPlas Rifle), but I can't figure it out. I would like to just re-use the scope on the Laser Rifle, but when trying to find the texture and meshes it uses I come up with nothing; they must be hidden.

So, how and where do I find the textures and meshes to re-use, and
Do I need to do anything other than set those as the same for the new mod and add it to the MultiPlas mod list? If so, what?

All of the game textures and meshes are kept in archive files called "bsa" files. Have you unpacked these using FOMM? Install FOMM, right click on the mesh bsa file, and unpack it. Now you should be able to see the meshes. Same for textures.