Transparent Cutout Fill Empty Face

Not sure what this would fall under, but I’ve begun writing a shader for a spherical area of my game, which uses the distance from a point to render a different texture (so parts of the object inside of the sphere will be visible, whilst those outside will not).
See here:
2249915--150323--Screen Shot 2015-08-14 at 00.18.50.png
This is all well and good, and works as I want it to - however the problem comes when looking at it from outside. Obviously the faces at the back are transparent as per the shader, but this allows the player to look inside the object. What I would like is advice on how to render a back face which would fit with the object’s orientation - the only way I can think of this possibly working is as the area is a sphere making parts of the sphere which intersect with the object textured… how this would work is what I need your help with.
Thanks everyone, sorry for the long question (note: I am using a surface shader for this so far).

Would just like to point out that I’ve figured where to go with this, which is Stencil Buffers, but still struggling with the implementation. Though the unity documentation has a similar enough example (Unity - Manual: ShaderLab command: Stencil) it isn’t really very well explained. Hopefully someone could help.