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If you don't mind using OSL (that is, setting the render to Cycles/CPU, and checking the 'OSL' option) then you can look ahead down a ray, using a script.

This script returns the Pass Index of the next material encountered, continuing in the direction of the incoming ray, (so it won't, without improvement, take refractive/reflective deflection into account, and the lens is a single plane, without thickness.)

#include "stdosl.h"
shader NextIdx(  
  output float nextIdx  = 0.0    
)
{  
  if (trace(P,-I,"mindist",0.00001)){
    getmessage("trace", "material:index", nextIdx);
  }  
}

.. and can be used in a tree something like this, to make the 'glass' of the lens:

.. where the eye material has an Pass Index of 10, and everything else has an index of 0...

producing..

so in this example, the 'glass' is transparent to everything has a material Pass Index greater than 5, and diffuse grey to everything else.

If you don't mind using OSL (that is, setting the render to Cycles/CPU, and checking the 'OSL' option) then you can look ahead down a ray, using a script.

This script returns the Pass Index of the next material encountered, continuing in the direction of the incoming ray, (so it won't, without improvement, take refractive/reflective deflection into account, and the lens is a single plane, without thickness.)

#include "stdosl.h"
shader NextIdx(  
  output float nextIdx  = 0.0    
)
{  
  if (trace(P,-I,"mindist",0.00001)){
    getmessage("trace", "material:index", nextIdx);
  }  
}

.. and can be used in a tree something like this, to make the 'glass' of the lens:

.. where the eye material has an Pass Index of 10, and everything else has an index of 0...

producing..

so in this example, the 'glass' is transparent to everything has a material Pass Index greater than 5, and diffuse grey to everything else.

If you don't mind using OSL (that is, setting the render to Cycles/CPU, and checking the 'OSL' option) then you can look ahead down a ray, using a script.

This script returns the Pass Index of the next material encountered, continuing in the direction of the incoming ray, (so it won't, without improvement, take refractive/reflective deflection into account)

#include "stdosl.h"
shader NextIdx(  
  output float nextIdx  = 0.0    
)
{  
  if (trace(P,-I,"mindist",0.00001)){
    getmessage("trace", "material:index", nextIdx);
  }  
}

.. and can be used in a tree something like this, to make the 'glass' of the lens:

.. where the eye material has an Pass Index of 10, and everything else has an index of 0...

producing..

so in this example, the 'glass' is transparent to everything has a material Pass Index greater than 5, and diffuse grey to everything else.

If you don't mind using OSL (that is, setting the render to Cycles/CPU, and checking the 'OSL' option) then you can look ahead down a ray, using a script.

This script returns the Pass Index of the next material encountered, continuing in the direction of the incoming ray, (so it won't, without improvement, take refractive/reflective deflection into account, and the lens is a single plane, without thickness.)

#include "stdosl.h"
shader NextIdx(  
  output float nextIdx  = 0.0    
)
{  
  if (trace(P,-I,"mindist",0.00001)){
    getmessage("trace", "material:index", nextIdx);
  }  
}

.. and can be used in a tree something like this, to make the 'glass' of the lens:

.. where the eye material has an Pass Index of 10, and everything else has an index of 0...

producing..

so in this example, the 'glass' is transparent to everything has a material Pass Index greater than 5, and diffuse grey to everything else.

If you don't mind using OSL (that is, setting the render to Cycles/CPU, and checking the 'OSL' option) then you can look ahead down a ray, using a script.

This script returns the Pass Index of the next material encountered, continuing in the direction of the incoming ray, (so it won't, without improvement, take refractive/reflective deflection into account)

#include "stdosl.h"
shader NextIdx(  
  output float nextIdx  = 0.0    
)
{  
  if (trace(P,-I,"mindist",0.00001)){
    getmessage("trace", "material:index", nextIdx);
  }  
}

.. and can be used in a tree something like this, to make the 'glass' of the lens:

.. where, in the example, the eye material has an Pass Index of 10, and everything else has an index of 0...

producing..

so the 'glass' is transparent to everything has a material Pass Index greater than 5, and diffuse grey to everything else.

If you don't mind using OSL (that is, setting the render to Cycles/CPU, and checking the 'OSL' option) then you can look ahead down a ray, using a script.

This script returns the Pass Index of the next material encountered, continuing in the direction of the incoming ray, (so it won't, without improvement, take refractive/reflective deflection into account)

#include "stdosl.h"
shader NextIdx(  
  output float nextIdx  = 0.0    
)
{  
  if (trace(P,-I,"mindist",0.00001)){
    getmessage("trace", "material:index", nextIdx);
  }  
}

.. and can be used in a tree something like this, to make the 'glass' of the lens:

.. where the eye material has an Pass Index of 10, and everything else has an index of 0...

producing..

so in this example, the 'glass' is transparent to everything has a material Pass Index greater than 5, and diffuse grey to everything else.