The table mendixsystem$version is not accessible through mendix nor is there an Mendix function available in the api of mendix. See:
I have submitted a support request to Mendix.
See this post. I suggest filling a feature request. The more requests the more it is likely it will be realized.
You can use the form loader widget to load a form in your theming. So just create a dataview which shows your current version. Maybe you could use some java to retrieve the current revision.