I suppose that could be done as a precaution (to glue something to the bottom and then plane it off). The thing that makes cutting toward the mouth from the inside of the plane less convenient is that the wear is fairly long (the whole thing looks more like a y than a V, and working from inside to out is not very convenient. Drilling from the outside, you only need to drill just enough to be able to get something (a narrow float or a saw) into the mouth to open it up.
Not sure how a professional maker would do it, there's a million ways to get it done, I guess. I just like to mark the sides of the plane to about what the shape of the opening/wear will be so that I can see it while I'm drilling, and then layout a rough estimate of the mouth so that I know the drill isn't wandering anywhere right at the mouth.