Hi,
Sorry for being late to the party, but I think what you want to do is quite achievable with RaySwitch and/or alpha mode override.
First, I think you should use thin mode for glass for optimal render times.
Then, you can use a RaySwitch material (without reflections, simple gray material) for the Global Illumination of the outside objects, to get the shadows from them.
This way they are still transparent (black) to the alpha, and you can easily composite what is outside, through the glass, later on.
Of course you can get different render elements and try complex compositing, but I am always for keeping things simple.
You can as well render everything seen through the glass as real (with correct, physical material), and then set the glass alpha mode to always black to completely change what is directly seen in the glass (or overlay with different opacity).
This really depends on the objects outside of the glass and the type of compositing to be done too, of course. But I don't think complex solutions bring improvements in this case.
Please see the attached image for more details.
