I couldnt find a thread for the style of brides and guests at royal civil cermonies (sorry if there is already one) but since the wedding in luxembourg features a civil element - i thought it would be good to look a previous styles.
The Royal Order of Sartorial Splendor: Wedding Wednesday: Civil Wedding Dresses this blog features a great summary of some royal civil dresses.
Some others like Maxima have used their weding dress for both religious and civil cermonies as they were the same day