Unless there's some physical reason you can't run physical conductor there's every reason not to do this.

This setup proposes to use 100mw/cm^2. That's just not acceptable unless you're sure no humans or animals are going to cross the beam (or even it's sidebands!). Even a horn has diffraction sidebands, and unless they're all at least 8dB down in all situations that's bad. The laser trips won't cover this.

This exceeds the safety standards established by the US Navy back when they were dealing with their high powered radars for the first time. They found that anything over 10mW/cm^2 can cause enough heating in animal eyes to cause clouding of the eye. Humans' deep set eyes and brow ridges help (depending on freq) but 10mW/cm^2 can still cause health issues. The measured e-field for that powerlevel, depending on wavelength, will almost certainly be above the electrical field limit for exposure in the USA. I hope that's also true in NZ.