Cycles is quite physically accurate, depth of field in reflections works like it would outside them.
All you have to do is account for total actual "distance to the object reflection".
Say if an $Object A$ is at 2 unit from the reflecting surface $Mirror$, and the mirror is at 4 units from the $Camera$. Then a ray traveling from the camera to the "object reflection" will have to travel the cumulative distance from the $Camera$ to the $Mirror$, plus the distance from the mirror to the reflection beyond it.
This would be equivalent to the cumulative distance from the camera to the mirror plus from the mirror back to the object, hence $2 + 4 = 6$, which would give us the correct value to insert in the camera Focus Distance parameter of the camera.
In following example the camera is 4 units away from the mirror, the red cube is 2 units away from the mirror and the blue one 1 unit.
Each cube has 0.4 units width, you can see each side independently focus for each 0.2 units increment of Focus Distance parameter.