Add new a data type, utf8varchar that can harbour UTF-8 data.
The max length would be 2666, since a UTF-8 character can take up three bytes. (Or you could set the limit higher, with risk for truncation errors.) Since UTF-8 characters are variable length, it does not seem very useful with a utf8char data type.
UTF-8 literals could be marked in analogy in the propsal about typed literals in http://connect.microsoft.com/SQLServer/feedback/ViewFeedback.aspx?FeedbackID=356470