Is there a way to tag text in word processing docs with metadata?

I am looking for a way to create a sort of “hypertext” where I can select some text in my document and append some invisible metadata that could later be used to see for example related items. I imagine it to operate as Tags work in blogs and other web systems where keywords can be appended to objects and consequently you can filter out everything but the tag you later select. Does anyone know of anything like this?

I am looking for an existing solution, and I don’t want to get into anything too fancy. I think I might be able to imagine XML tags sort of working somehow.

About all this, if what I am looking for doesnt make sense, and there’s a much better way to do what I want I am quite open to suggestions…

Thanks!!