I see your point.
However, the influent blocks (of type state vector, i.e. Municipality, Industry and Truckload) are different from any other block in that they are not associated to a model which resides in the model library: instead the model is created, at the project level, by executing the Influent tool / wizard.
So far (2014 and upcoming 2016), the only quantities (OBJ's) that can be [b]defined via the Fractionation diagram[/b] are plain [b]parameters[/b] and [b]algebraic state variables[/b]. No input interface variable (and set to manipulated) can be defined - which is what you would need in this case.
You would also need to modify the icon itself, to expose the manipulated variable through a terminal - but this would be a minor issue.
The main one is that you cannot define a manipulated interface variable.