During a St. Louis Comic-Con panel a fan asked Nathan Fillion whether or he wanted a part in Guardians of the Galaxy, since Joss Whedon is so heavily involved with the MCU:
“[Want] to get a part?” Fillion said with a smirk. “Or maybe did. I’m just saying maybe. Maybe it’ll be a surprise. Maybe check the credits of the movie when you watch it.”
Check out the video below and judge for yourself, but I think Fillion’s tone makes it a pretty solid bet that he’ll be appearing in Guardians. The way he says check the credits seems to imply that he’ll be unrecognizable. An alien in a bar or jail or something of the like. They may be a chance that he means a credits scene as well.
Guardians of the Galaxy is directed by James Gunn from his screenplay, with a story by Nicole Perlman and Gunn. The all-star cast includes Chris Pratt as Peter Quill aka Star-Lord, Zoe Saldana as Gamora, Dave Bautista as Drax the Destroyer, Vin Diesel as Groot?, Bradley Cooper as Rocket Raccoon, Lee Pace as Ronan the Accuser, Michael Rooker as Yondu, Karen Gillan as Nebula, two-time Academy Award nominee Djimon Hounsou as Korath, Academy Award winner Benicio del Toro as The Collector, Academy Award nominee John C. Reilly as Rhomann Dey, and six-time Academy Award nominee Glenn Close as Nova Prime.
Watch at around 38:48