I went out and took pics yesterday.

As you can see the access plate takes up quite a bit of the mast width. It is about 4.5 inches by about 6 inches. The mast itself is about 7 inches wide at the base. The mast needs a good painting... which I plan to do when I replace the chainplates.

Now I have no idea how long the plate has been there, or if it came with the mast originally. But it seems plenty strong, and I don't have any compression problems. Perhaps the thickness of the mast is such that the plate is not really an issue overall. The mast is deck stepped.

This obviously may not be a solution for everyone.