The team will get to take 30 players to Australia for the trip. However, only 28 players will form the actual roster for the two "regular season" games. Three of those, selected by the Diamondbacks, will not actually be eligible to play in those games - the most obvious candidates for those spots would be the members of the normal rotation, not playing in the Australian games. They may not even travel to Australia, thereby avoiding any potential of them suffering jet-lag or any other hangover from the trip. There won't be any scheduled Cactus League games for Arizona while the team is away (between March 16-26), but those starters could work against our minor-leaguers.
The other two spots beyond the 25-man roster would likely go to pitchers for the exhibition game being played against Team Australia the night before we face the Dodgers. It would not surprise me if both of those were starting pitchers, and we simply see one of them work five innings, and the other four innings, so as to keep the "real" bullpen rested for the meaningful contest the next day. Sorry, Team Australia: you may not even get to face major-league pitching, but blame MLB for their scheduling.
With regard to who makes the 25-man roster, that certainly will not be the same as for the American Opening Day on March 31. Technically, the team could carry just two starting pitchers, opening up three spots for use in other areas - either additional relief pitchers or bench players. That might be a bit risky, however: one Cahill-esque comebacker to the mound in the first game, and your bullpen suddenly finds itself having to work eight innings that night, plus whatever needs to be done the next evening. It wouldn't surprise me to see Randall Delgado kept in a starting role before the trip, partly to fill the role of emergency starter if needed - and he is indeed starting Thursday.
On the other hand, for the second game, we can certainly empty the bullpen if necessary, since there will then be nine days to recover before the next game that counts. I think we may well see another left-handed reliever added, because the "standard" bullpen we have, will contain only Joe Thatcher in that role. Another southpaw would expand the possibilities again left-handed Dodger hitters, particularly Adrian Gonzalez, but also Andre Ethier and Carl Crawford. Possible candidates for that additional spot include Eury de la Rosa, Joe Paterson and a waiver-wire pickup last month from the Chicago White Sox, Santos Rodriguez.
However, if I was a betting man, the name on which I would wager there, is probably non-roster invitee Ryan Rowland-Smith, assuming his spring-training performances prove up to snuff. Looking at 2013 numbers, there likely wouldn't be much difference between him and the names mentioned above, In terms of performance,and the positive PR obtained by playing an Aussie on home turf would be enormous. Don't forget, these are "home games" for the team, and maintaining that advantage 7,800 miles from Chase Field would be nice. [Speaking of which, I'd like to thank Zach Greinke for his recent help in this area.]
On the position player side, I would probably look at adding Tony Campana. Sure, he's a one-dimensional player to a large extent, but that dimension can be very useful late in a close game, where a stolen-base or two can prove important. An alternative possibility might be to add a third catcher, presumably taking both Tuffy Gosewisch and Henry Blanco. We occasionally have seen this role present in the majors, generally once rosters expand in September. It gives additionally flexibility, allowing you to pinch-hit for your catcher later on, without worrying about what might happen if his replacement is then hurt.
If the team opts to use Delgado in that emergency starter role, we may indeed see both Campana and a third catcher added to the roster. Or the team may instead decide to go with an additional bullpen arm. Given we still don't even know who are scheduled to start these games - my money is on Corbin and Cahill - there's plenty of time to decide. The 28-man roster, and the names of the three inactive players need to be submitted to MLB by 7pm Arizona time on March 21, six hours before the first game is due to start. I imagine the Diamondbacks will keep their cards close to their chests until that point.