I don't really get what you're mean, vanilla texture packs allows to change all interface aspects (inventory, chest, icons, item textures...)
And the interface added by other mods, if its a image then that can also be changed due texture packs.
Optifine adds even more customization to texture packs, like alternative random variations, connected textures, etc...
So... I guess you're talking about adding more stuff to the normal interface like minimaps, armor stats &stuff?
All this stuff is added by the mod BetterPVP and I think you can customize the interface, not sure in 1.8.9 position.
Probrably its more like, you want to add extra elements cause you're an streamer or you seen some streamers have gifs and alerts all arround?
Thoose elements are not added on the game view, if it was the case, the streamers will not be able to see properly the game.
Thoose are added by the streaming sofware they use.
Probably is more like you're making a map with mods, an adventure or running a server and want to add some images over player screens like "welcome to the lobby" or something like that tiggerred by a command?