Yes, it is not forbidden. But it may be hard for many sensors to detect.
Still is it common to use PET bottles of water in the community. I would never use it, but it is easy to find and many uses it. I would say it is better to fill them with sand, like they did in Brazil. But water is also common.
I have written guidelines for field designers where I state: "constructing obstacles rounded or soft is not nice since most sensors will give inaccurate reading when the signal is bouncing against the surface of the obstacle."
I will add transparent obstacles to that sentence.
// Fredrik Löfgren, TC and OC member 2017