In Godot 3.0 there is a hack to draw first-person meshes, which is to have the model next to your camera and check
no depth test in its material. This will draw the mesh on top of everything else, however it's limited to a single mesh.
In 3.1 there is a plan to add render layers, so you could render anything in a separate layer to have it on top (from Reduz on IRC).
For other players, if your game is multiplayer I guess you can simply not instance the same one, since you certainly know which player is you and which player is not you.