First you have to completely decouple the cavity from it's surrounding environment - especially the RF-field has to be generated by an apparatus fixed to the cavities frame of reference. Also the power has to come from a source connected to this frame of reference.
For me this is Physics 101.
As otherwise your system is not even closed. You are just neglecting that you are pumping LOTS of energy into an open system that you are measuring while assuming it to be closed -> thus leading to wrong conclusions (measuring a change in impulse in one part of the system)
For me this in line with uBeam and the like.
Whacko-science wherever one looks.