I am assuming that you are combining real video of a lake, and want to add a blender object into the scene, and are using a plane with an environment map to reflect that object, but you want to be able to see through the plane, and still see the reflection…
The method I would choose is to have the envmap on the plane modify both the color and the alpha.
press both the [Col] and [Alpha] buttons in the material buttons for the envMap texture channel.
The brighter the color in the env map, the stronger the alpha. The Var slider will allow you to control how much the envMap affects the alpha channel.
Oh, and I would suggest that you set the material color to black, alpha to 0, turn ztransp on (required), and use add instead of mix for the texture, but it doesn’t really matter.