I think they'd do it.
You make a film with inspiration of FP, you have the whole universe available to you, you have a white hot Gal Gadot and maybe Jason Momoa after next year, and you have a perfectly legitimate reason to include Batman, and the actor happens to be a fan favorite, and Cap 3 just made an extra 200-300M because of the CW approach, and constant reports of WB looking to team up Flash with Cyborg, or worst scenario to chuck him in a Titans film, I think they were always looking to support him with other characters to boost the brand, would be nice to see some data on GA awareness and enthusiasm to see a Flash film.