If you consider that a value type
cannot inherit from another struct
http://msdn.microsoft.com/library/ah19swz4%28v=vs.90%29.aspx
it makes no sense talking about variance for them. If you box it into a reference type what type do you think to use, other than object? You should define an implicit or explicit cast to a reference type but for what is worth?
Variance is about polymorphism and inheritance. Consider that when you assign an int to a variable of type long you have implicit cast: there's no inheritance relationship between the two types.