Thinking about this question after reading Tom Sawyer and listening to Engel's lecture on Twain's life, I think Twain's definition of morality would include acting for the good of humanity but still having fun. I don't think Twain was Kantian in his ethical makeup--Tom's "white lies" and devious behavior throughout the book certainly don't reflect a universal law mentality.
I think Tom envelops many of Twain's own characteristics--for example, Twain's dislike of high society is mirrored in the character Huck Finn. Similarly, Tom's view of morality is probably pretty similar to Twain's. Though many of Tom's antics are less than moral, he still does the right thing when necessary. For example Tom defends Muff Potter during the murder trial, even though Tom is scared of Injun Joe's reaction. Tom also comes to Becky's rescue over the ripped anatomy book and takes the punishment for her. Tom lies to Aunt Polly on several occasions, but never in a dire situation.
I think Twain would describe morality as doing the right thing when it counts and treating everyone with respect. Being of genuinely good moral character and acting selflessly, I believe, would be part of Twain's definition.
