Captain Apollo meets Luke Skywalker! :-)

Probably the closest fans ever got to a crossover between Star Wars and Battlestar Galactica was The Streets of San Francisco episode "Innocent No More" (02/24/1977).  In this episode (filmed after Star Wars: A New Hope - but aired 3 months before the film was released), Mark Hamill guest-stars as a 16 year-old juvenile delinquent.  Unfortunately, Richard Hatch (Inspector Dan Robbins) and Mark Hamill don't interact at all in this episode - they're together only in the final scene where Robbins saves Hamill's character from being shot but they never speak to each other. Here's a photo from the episode:

Richard Hatch and Mark Hamill on The Streets of San Francisco
("Innocent No More")

The Streets of San Francisco is copyright (c) CBS / Paramount; photo used for illustrative purposes only.