They don't work like you think. When you see a huge number, that is the top line cap at what the person will get paid. "Book deal" is not the same as "advance payment". The agreements are complicated and come with a 360 service agreement where you go on tour working for the publisher, speaking at events where the publisher collects your speaking fee etc. Options on future books if the lower "advanced cash" is not met.
The big headline book deal numbers are part of the promotion to create hype. Its possible this could be a vehicle for some illicit payment, but doing it this way would be pretty inefficient from a time/tax point of view as the payment would be treated as ordinary income and there are so many hoops to jump through.