I think Lesnar would work with WWE again for Wrestlemania.I believe he earns about 500,000 for every UFC fight; he would earn about 2 million dollars for one choreographed wrestling match.Also,the MMA fans see WWE for what it is - an entertainment show - so he wouldn't lose face IMO.
I can certainly understand why Dana White wouldn't want Lesnar working and losing in another company - fake or not.But you can bet McMahon would love to book it,because it would guarantee one million buys.