I wouldnt attempt flipping it in transit for this very reason. Its either up or down. I installed it last night, there is no way for them to see the device behind the bracket unless they suspect and physically dismantle the back end of the bike.
Does it flip up or down? If it flips up it might still be visible to the cameras.
When will you flip it then? When you leave home or when you enter the highway? In either case if they see you on the e-toll road without a visible number plate they will come for you.