The xpath language in Mendix is a language inspired by the xpath standard for xml, but not trying to implement to official standard. Xpath in Mendix is translated into OQL so it can be used for relation data in your database. This is unique to the Mendix xpath implementation and means that we cannot rely on an existing xpath library, and cannot easily upgrade it.
We can add useful constructs from the xpath standard if they make sense for database data. Your example would make a good additional, i'll add it as a possible improvement on our roadmap.
You should upvote this idea, as it's a feature request.
in the same vein, there is also the problem of apostrophes. The escape character is not supported by XPath in the current version of Mendix.