btGhostObject->getOverlappingObject (index);

"The btGhostObject can keep track of all objects that are overlapping it.  By default this overlap is based on the AABB. This is useful for creating a character controller, collision sensors/triggers, explosions, etc."

I started by adding collision filtering using bit masks and then completely refactored the collision detection code for collectables using a btGhostObject.
"A more efficient way is to iterate only over the pairs of objects that you are interested in. This can be done using a btGhostObject."
So much simpler this way: