I guess it really depends on the type of person you want your King to be.  

The first questions I would ask myself is; 
-Is the King a "No nonsense" kind of guy? Would he be straight to the point and make the announcement at the beginning? 
-Would he still be on the straight forward side and have the courting Princes pick there brides to be first, then afterward make the announcement as a "We have one more unexpected alliance to announce" kind of thing. 
-Would he make the announcement somewhere in the middle of the event as a "lets see how this effect the mood" kind of thing?
-Does he have a flair for the dramatics. Announce that there was an unexpected situation that he will explain more at the end of the ball, to create an air of curiosity and entice the guests to stay through the entire event? 

If it were me I would either go for making the announcement after the pairs are made with his daughters, or I would go the dramatic route and wait until the end. But that's just me.