I cannot advice on the most appropriate settling model for low SRT / loading rates - hopefully other users will be able to assist you on this.
What I can suggest though is a practical solution to the issue you describe, i.e. "[i]how to automatically have the settling properties of the sludge vary with the sludge age[/i]".
What you actually need is to control (via a custom control model) the settling properties based on e.g. SRT.
So, the first thing you need is to implement such control model in MSL.
But, in order for the settling properties to be "controllable", they are to be defined as [b]manipulated variables[/b] (= input interface variables, flagged as [i]manip[/i]) - and this is generally NOT the case, if I recall well: e.g. 'v0', 'v00' and 'r_P' are all [b]parameters[/b] in the Takacs model.
So, you need to modify the base implementation by: i) moving the definitions of these OBJ's to the interface section of the model (and adapting the definition, by e.g. indicating the causality, terminal, etc.); and ii) modifying the equations in which they are used by replacing "parameters.[NAME]" by "interface.[NAME]"