Perhaps this is a brilliant Paul Heyman scheme to promote Lesnar's book to both the MMA and pro wrestling community, setting up both sides at the same time for maximum exposure.
Take for instance, instead of a wrestling match, he makes a Mickey Rourke-esque appearance that is promoted through third party media in the buildup. So he doesn't actually wrestle in a match, thus bypassing Dana's contract while also retaining his inroads with the MMA community. Afterall, he's just promoted making an appearance on an entertainment show. Obviously, WWE would do a buildup of teasing a physical confrontation involving Brock with someone (doesn't have to be Taker; someone like Punk or Miz or Sheamus would work and give them some mainstream rub) and the payoff could be Brock F-5ing the guy at Wrestlemania, similar to Rourke punching out Jericho.
At the same time, he would be getting the UFC push for the book with their business model of his comeback tour to the heavyweight title.
In the end, he gets Vince's television exposure for the book as well as the UFC's exposure as a main event attraction.