Hmmm, I can't think of any way to make an object treat raytraces differently depending on whether they are coming directly from the camera lens or from somewhere else.

But maybe, just maybe, there is some shader node that returns the raytrace recursion number. Then the object could be made transparent for the primary ray and opaque for all subsequent recursions. That might do it.

Any raytrace node gurus hanging about?

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)