It may be better to tell us your end goal. Do you want to keep the sheen off the glass, or are you trying to get a reflection?
If you want a reflection, don’t need the glass. Shoot whatever you want the reflection to be and map it where the key will be and drop the opacity. You will want to distort the image though. Also if your camera rotates around the window it may give away the reflection doesn’t move correctly. You can animate the reflection to move opposite from your camera move to give the illusion though.
If you just want the sheen and light reflections, then put glass over your key. You only need a key to get good edges so the midle doesn’t matter. After you get your key, use that key to isolate the original footage. Turn it greyscale and play with levels and transfer modes to map your reflections and light hits back onto whatever you comped in.
Chris Smith
https://www.sugarfilmproduction.com