How good a sport is Robert Earl Keen? When he realized that “Merry Christmas From the Family,” his 1994 tune about a motley clan that gets along well enough during the holidays so long as the margaritas and homemade eggnog keep flowing, was well on its way to becoming the kind of song that can define an artist’s career — at the expense of anything else they’ve written, even — Keen shrugged and started touring his core markets in the Southwest every December without fail. Besides, Keen’s 2016 has already been especially productive. In February he released his first-ever full-length bluegrass effort, Happy Prisoner: The Bluegrass Sessions, featuring guest stars like Peter Rowan and old pal Lyle Lovett, and last month saw the launch of Live Dinner Reunion, the long-awaited sequel to the album that remains Keen’s best-selling record, 1996’s No. 2 Live Dinner.