Alpha channel for selected objects in reflection.

The task is to render only defined objects in reflection with alpha channel which will separate that objects from other objects in the same reflection (unselected or udefined).Undefined objects of the reflection should behave as matted ones relating to defined. All means I know will not acomplish such a task.
I know how to render alpha for all as one reflected objects to separate them from reflected background but the task is more complex. So, possibly it is about a script.

Thank you.