As a stage manager, I share your frustration. Unfortunately, we do not always have much say in where the company / understudy boards are located. If you see multiple shows in the same theatre, you will likely note that the boards from show to show are generally in the same location.
No one likes making a pre-show understudy announcement. There will ALWAYS be an audible groan from the audience and it isn't fun for anyone. We make sure to have the understudy slip(s) out for the front of house staff to stuff programs before they report an hour before the show, but after that, to a large degree, it is out of our hands and we trust that it will be finished or the house manager will let us know otherwise, and if an actor calls out after that, we don't have time for an insert... That 30 minutes before house open is a tight window.