Hmmm... I don't know,
@Britsimon has a better idea of the possible delays involved in unlocking DS260s for something like this, but it sounds very late to me to be honest. Remember you get interview notification about 6-8 weeks before the actual interview, and the spouse needs to have been added on to the application and his/her DS260 processed to be added on to the interview. Even if you add and they process instantly (i don't know if that's even possible) after your marriage, that's only going to work for an August or September interview. If they schedule you(r spouse) earlier, the interview will be scheduled as single and that's going to be a major PITA to work around for most consulates. If someone is not on the interview schedule, they are not allowed in and cannot get a visa.