Thanks for the detailed answer, hansmex.
I basically want to make a loft (if this is the right word) ending with a terrace under the roof, separated by a glass wall. So none of the options you described are doing it for me.
It would've been possible with a right triangle window or frame, but, as you say, that's impossible. It's only a small inconvenience.

Maybe in the future releases it will be possible to vary the transparency of independent wall units...