Whether or not hidden text is printed is an application setting on each user's computer. It is not a document setting.

You do not have control over that setting on someone else's computer. The suggestion of a light watermark is the best you are going to be able to do.

You can make it difficult to change or erase the watermark, but not impossible.

I generally have display of hidden text turned on when I use Word. Your method would not hide it from me. I suspect the same is true of at least some of the people who would be using your document.

If you have the Office Information Rights capabilities, you could likely do something with those. I have never used it, so do not know.