I bought a payware plane at the .org store a few weeks ago. I like the plane, but the plane was not protected in anyway, shape, or form.
Any ten year old could have uploaded the plane to a torrent. A serial code system did not exists on that file. I also believe that some of the files on your store do have a serial system already? I don't know for sure how hard it is to crack one of those right now. But if those serial code systems work, then it should not be to hard to make a demo mode right?
Maybe you can have a online plane model viewer, where you can walk around and inside the model online. The model viewer is online so you should'nt be able to crack it without a lot of work. In the online model viewer you could also look at the different liveries. You could make the model viewer in say Unity3d which is a free 3d game programming software, it can also do online things.