What's more, the enforcement seems to be purely based on radar/transponder readout. In a paraglider, or possibly even in a plastic fantastic microlight, with the transponder turned off, you would be virtually invisible.
Not entirely true. Being a radar maintenance engineer by military training, I can assure you that a paraglider is perfectly visible on a radar, and anything with metal parts in it, even if these parts are only a few centimetres in size, is visible still better. The question is whether a given blip is assessed as a potential threat (discrimination methods may vary) and whether a timely response to this perceived threat is effected. In fact, fooling even fairly sophisticated means of defence may not be too difficult - fortunately, people with enough intelligence to orchestrate it properly hardly ever become terrorists (although, from my own experience, people in charge of such defence are usually not terribly bright, either). The modern nuclear industry, however, doesn't really take such risks lightly, and places a great deal of emphasis on physical protection - so that, for example, a backpackful of TNT may simply be insufficient to breach the containment.