How do I access the members of a TMap in a predicate for ValueSort()?

They do not need to be lambdas, but I believe it is the easiest method.

In standard C++, the lambda syntax actually produces a class with an overloaded operator(). In this case you could add a

class FMyValueSort
{
    public:
        bool operator(float A, float B) const
        {
            return A < B;
        }
};

somewhere (either in your .cpp or .h file) and do your sorting like so

FMyValueSort MyValueSort;

MyMap.ValueSort(MyValueSort);