That would work indeed and actually looks quite handy. But you could better use the onCreateDialog() and onPrepareDialog() in Activity and use showDialog() accordingly. You should then override showDialog() to not only take the id of the MessageBox (if there's only one possible dialogbox which enables you to easilly handle messages, then this id could be left out) and the message itself. In onPrepareDialog() you then update the string which will be shown in the dialog created in onCreateDialog().
This way, it's more Android, yet should be as easy to use as your proposition, and the main advantage is that Android handles the configuration change, so the dialog will stay visible if you change orientation.
Though I just looked it up, onCreateDialog() and onPrepareDialog() are protected members, so the use of this Android based approach would require more input from the developer, so that makes it less attractive, though, this implementation could be done using 1 line for each on..Dialog().
Hope you get what I'm trying to say