There's currently no functionality like what you described because there's no context for it. There are some variants of XML that we do light tagging on, such as docbook, because there are clear examples of declaring a symbol and referencing it in other locations in the file.
However, in your example, there's no context for what you want it to do. For example, SlickEdit would have to know that <int name="myVar"> meant that you were declaring a symbol named "myVar" that will be referenced in other locations. Since it doesn't know that, it's not identifying "myVar" as a symbol and won't know that there's a definition for it.
