Somebody helped me out with this once before, but for the life of me, I can’t remember what the solution was and I can’t find the thread. I remember it was a really, really simple fix, though.
The basic idea is to have an object that is see-through, but anything inside of it is invisible.
Imagine you’re rendering a star-gate and you’re looking at it edge-on. The spaceship flies in one end but obviously doesn’t come out the other. But if the camera were to move so that you were looking into the gate, the ship would still be visible. It’s sitting inside of a half-sphere that, from one side, cannot be seen into, but anything behind and outside it still renders.
I know there is a way to do this, and I think it’s just a material or map that’ll do it. Someone gave me this solution once before, as I said, but for the life of me, I can’t remember.