Thinking of adding this in a scenario, has anyone done anything similar? Just narratively, or maybe use Mass Combat rules...

Cannot say I've done anything too similar, but I think I would play it out as a chase. So, competitive athletics checks to determine whether or not the bulls close the distance, and if they do they can attack. Then you can throw obstacles at the characters and players will probably come up with their own ideas how to escape.

You could also use the running of the bulls as the backdrop for some other sort of action scene. Maybe the PCs are chasing down an enemy spy when he dips into the main route of the bulls to try to escape them. Then it becomes a chase against the spy, but the bulls become an additional obstacle. Maybe threats can lead to wounds on a 1-1 basis, or for 3 threat or a despair, the bulls inflict a critical wound. (I mean, it's a very dangerous place to chase down a spy!)

I'd probably run it as a skill challenge. Not an official rule, but The Dice Pool podcast has an excellent episode on how to run them in Genesys. I've used this method many times and it's a really fun way to craft a scene like this.