I'm reopening this old question, just to see if something changed (I think this was mentioned in one of the recent Mendix posts). Can constants or simple properties of an application be changed at runtime?
Not at runtime, only when starting the application.
Constant definitions (name and default value) are always configured in the modeler. However, you can (and should!) provide different values depending on which environment you're running.
In the standard cloud you can configure them from the portal. Assuming you're using m2ee-tools, you can configure them under Linux using m2ee.yaml
For Windows, there's a service console that allows you to edit these values, under the "application constants" tab.