Well, you could do it from the front ,you just need the right tools and don't screw up. I cut these with a vibrating cut saw all the time. I try to stay away from a grinder, or a rotozip because they can wander.
They do also make even larger cover plates, you just need to make sure your fiberglass surround will work with the cover. Some surrounds have bows in the wall that you set the plate against, and the weatherstrip does not compress enough....with all these decisions..you see why the pro's get so much $$
The good thing about the plumber doing this, is they own the tub if they get it wrong.
And you may actually be able to do this easier from the back, only you would know that from where I'm sittin.