Tai Yasue, co-director of the Kingdom Hearts series, just confirmed on the Square Enix Presents channel that the Mirage Arena segment in Birth by Sleep will NOT be playable online via PSN in Kingdom Hearts HD 2.5 ReMIX.
Yasue added that the reason for this was because they wanted to speed up the development of Kingdom Hearts 3, and spending extra time making Mirage Arena playable online would've taken from its development. Instead, they made tweaks to the single player mode of the segment, making it even more enjoyable in 2.5 ReMIX.
In the Mirage Arena, you can play as either armored Terra, Ventus or Aqua and play with other people in various different events, including the Arena Mode where you can fight enemies and bosses together with up to 3 players and Versus mode where you can fight up to 6 people. Along with that, up to 3 players can play the Command Board mini-game with each other and up to 6 players can race each other in the Rumble Racing in Disney Town.
Kingdom Hearts HD 2.5 ReMIX arrives on October 2nd in Japan, December 2nd in the USA and December 5th in Europe exclusively for the PlayStation 3.