You're unlikely to be told that you aren't allowed to do this or that, unless it's a safety violation of some sort. The idea that you can describe jobs to the letter and everyone is aware of what's written there and only does that is absurd. What's in the job descriptions protects you against abuse if someone makes you do things aren't paid for trained for, capable of, etc. It's a backstop. It doesn't prevent you from doing other things. In fact doing extra is a basis for promotion, just like it works in non-union shops. That's what how I've seen things working in a unionised university I have access to.

In any case, if a union card comes to my desk, I'd get the power first and worry about these details later. At least someone would ask me how I want these things to work, instead of telling me with the only alternative being to leave the company or be fired.