Those sections and elevations did suck alright.
Ultimately the single-file structure with Revit solves so many issues, I just don't thank ADT could ever be made to work as well as Revit does. Mostly I just miss a lot of the small things from ADT, little commands, the many ways to do lots of little things. Like this beam thing.
Now, The suggestion to cut it with a reference plane does work, fast and easy. If you just do the cut operation on more than one beam at a time...
In the meantime, looks like opening is the way: build one and then copy them out.