1 game series plunks
Since Craig Biggio joined the Major Leagues with the Astros in June of 1988, the team has had to travel back to a city for a 1 game makeup series 8 times. Craig Biggio was hit by a pitch in just one of those games - on June 8, 2000 against the Dodgers.
The prior day, they were in Houston playing the twins, and the day after they were in San Diego, but on the 8th, they were in LA and Biggio got hit by Alan Mills. Since the Astros have played in domes during all of Biggio's years, all 8 of those 1 game makeups were on the road.
To answer Anonymous's question regarding double headers, Biggio has only been hit twice on days the Astros have played a double header, but he's never been hit in both games of a twin bill. Steve Woodard plunked him in game 1 on May 22, 2000 at County Stadium in Milwaukee, and Jimmy Anderson of the Pirates plunked Biggio in the 2nd game of the day, on July 28, 2001.