I have no problems with transparent, a bit bluish tinted, windows in my game. I just move all windows to the transparency (foreground) world, and windows show all shadows and lights behind them, and also cumulate (you can still see through multiple stacked windows and the color adds correctly too)! Cumulative transparency is no problem in deferred rendering engines, you just need to do it right.
In the mat file for windows I use blend=alpha, color=1,1,1,1, and the window texture has an alpha channel and is colored light blue. Framework must be used for rendering, because without it, you would have to rewrite it 1:1 with all the buffer and rendering logic, resulting in the same situation as using it.