Wow, missed the part where TypeTuples can hold values. I suggest looking into using Tuple from std.typecons.
What you need to remember is that you are not casting the tuple, you are casting the data in the tuple which creates a completely different tuple. It seems you are trying to use the type system to be sure of the class type at compile time so that you don't have null values when you cast to the derived type. If this is the case then note that it can't be done.
