Isn't the contract that all business must be conducted in good faith? I don't see any reason for Universal/Comcast to simply pound their feet and say no on a character that they're barely using, as much as we want to think that Universal is actually that cutthroat. Obviously the rights to Spiderman, Hulk, Iron Man, Fantastic Four (including Doctor Doom), Xmen, Captain America are an absolute no-go because, you know, they're prominently featured in attractions and eateries. But I can't see why Uni would refuse literally any deal that Disney approaches them with as much as I'm convinced Disney already contacted them about Guardians and worked that out.
We also have the fact that Comcast might be butting in on the Fox deal which actually may result in a selling of properties to Comcast, such as The Simpsons as it doesn't exactly fit Disney's brand. This could factor into the actual conversations between Uni and Disney. Remember that the attraction, much like Guardians, doesn't necessarily have to be solely tied around the superhero himself. The world of Wakanda could be the subject instead, thus making it less a Black Panther property and more of a "property set in the world of Black Panther". I don't see any reason to see that this could never happen.