I've played extensively with the 10 GHz Doppler radar motion sensors, and use them in an indoor intrusion detection system. The one that Parallax sells is about $35, but you can buy the basic radar front end from Chinese vendors at about $6, provided you are willing to build your own filter/amp for it (one LM324, a few passive parts, < $1.00).
I've found that these units can be adjusted for sensitivity to motion from about 3 ft. to about 25 feet. So you might be able to use one to essentially see all around one of your sculptures. However, they only sense motion, but even a small hand movement is readily detected. Since they emit a signal, it might be hard to place them so that they do not interfere with each other. Of course, they do seem to work fine on store doors that are even 20 ft. apart.
Don't know if that would work, but its one alternative to examine.
Good luck.
John Doner