I dont see this ever happening. I think both companies realize that this sort of thing would escalate, and escalate ugly. About 9 year ago, during the promotion of the first Avengers movie - WDW wrapped a monorail with a skin featuring the original 6 avengers (Ironman, Cap, Hulk, Thor, Black Widow & Hawkeye). I suspect Uni had lawyers visiting 1375 Buena Vista Drive in all of about 30 minutes the same day the wrapped monorail left the barn to ensure the train never went inside a theme park (it stayed on inner the resort loop). AFAIK, WDW has not promoted any other marvel movie on a WDW monorail since then. If Universal had a 3rd party driving the equivalent of billboard advertising into the land of purple street signs, I could see Chapek authorizing mobile billboards to drive up and down Universal and I-Drive within a day. Mud would start to sling in online and print ads....it would get stupid, fast.
I suspect there's some variety of understanding between the two companies to stay out of each other's way and allow the rising tide to benefit both ships - especially in the current environment. It's just not worth, IMHO.
While I think it was a dumb move, WDW's decision to let go of DME was not to relinquish an advertising advantage. it was to cut costs - plain and simple. Given the state of things right now, we're going to continue to see every benefit and incentive go away - until they need to bring them back.