I think this is the no brainer and easiest one of all to get. It's a complete shock White would just come out and say, "Oh, Brock is under contract so he can't do that."
Notice, that White and the higher ups could allow Lesnar to "entertain". Remember, all pro-wrestlers have one job, entertain the fans of the promotion. I mean, that's the whole goal, and well, money too.
UFC knows of WWE and knows it's power. It's obvious that without them, they couldn't be marketing so well off Lesnar. And look at the careers launched from the WWE for former talent besides Rock.
UFC averages in the 1.0s on Spike TV weekly, for any show they do. Their highest show watched was a PPV, noticeably with Lesnar. And it didn't crack the 3's, and it was a nicely stacked card.
Yet WWE consistently gets 3.0s on RAW and their PPV buys, while down from recent years, averages as much as their SmackDown show gets on TV.
Then you add in the fact that they are an entertainment company, with cross over fame into television shows on Disney, Nickelodeon, Fox, NBC, ABC, etc. People watch WWE, and also realize that this is a worldwide phenomenon, not just a US product.
Hell, the WWE just made their first trip to China recently. One man by the name of Alberto Del Rio said he went to six different countries in his first six months in the WWE on it's main roster. Showing the obvious, the WWE has fans worldwide that any company would kill to have things promoted on.
Why do you think it's so easy for the WWE to get guest hosts?
Hollywood sees it's popularity.
Vince McMahon came out and said that he believes UFC is a totally different product than WWE, one is real fighting and the other is sports entertainment, a predetermined form of fighting.
So Vinny Mac isn't asking for UFC to send one of it's more popular stars to the enemy's camp here.
UFC would get huge promotion for any type of appearance Lesnar would make for WWE.
All I have to say is, in knowing how big your company would profit, why not UFC?