Can you use namesake when refering to just someone's surname?
Eg If the persons name is David Chaplin, can you say: "unlike his namesake, Chaplin's efforts are nothing to laugh at." ?
You can if the preceding context makes it entirely clear. For instance:
"David Chaplin must have thought he was funnier than the familiar short man with the big shoes when he performed last night at the Miles o'Smiles comedy club. But unlike those of his namesake, Chaplin's efforts were nothing to laugh at".
However, to avoid completely confusing people who had never heard of Charlie Chaplin, I would probably qualify the reference to the namesake with an adjective or adjectival expression, such as famous, wittier or far better known. This device doesn't explicitly identify Charlie Chaplin, but it does give the reader some inkling of why the comparison is being made.
The term "namesake" generally means that the person was given that name on purpose, in reference to another person. In the case of a surname, unless it is part of a professional, trade, or stage name, I wouldn't assume that the correspondence is intentional and the word "namesake" is somewhat inappropriate.