Slightly tricky as the Master usually acts as a container for multiple other mashups. I think you have two options...
1) All of your child mashups need to have the same collection of Mashup Parameters. When you define your master, set the default content to be one of these mashups, you should then be able to bind to the mashup parameters of the child mashup. The binding will still work even when you change child mashups as long as the new child mashup has the same parameters.
2) Use session parameters, you can extend these from the default Entity and ZoomLevel by adding a ThingShape containing the properties you require to the UserManagment SubSystem.
Hope this helps