They book them through VIP tours. It's quite expensive, but if you have the money, you can have a private tour guide. But note: You do not get front of line access or any perks above regular guests besides having a personalized guide.
Now, to answer the question about "Frequent guests", I am going to say no. I don't think it should be offered. It creates too much of a power complex of "I've stayed here XXX times so I am entitled to it." It could create many issues. One of the things I really do love about Disney is the fact that once at WDW the hierarchies of the world do not apply. We all buy the same tickets, have the same choices of hotels, wait in the same lines, eat the same foods, have the same access to features such as Fastpass and transportation, etc. Nobody is above anyone else, and it works. The hierarchies that exist in the outside world do not matter inside the park, it is truly an escape from reality. How would Disney offering special benefits make them different than the rest of the world.....it may start to feel like an airport, with separate places for frequent travelers, etc...bringing the outside world in....which none of us want.