A famous quote applies more often than not... "Perception is reality". What it means is despite the way things really are, you have to deal with other peoples perception because for them it is reality. Regardless of all the good intents to encourage one type of game experience, you will have to deal with the idea that you are showing favoritism to one group of players. Xillix through out a few examples of how an approved guild could benefit by new game content.... what about all the other guilds? Why aren't those events available to every guild in the game?
If you want a suggestion on how to make it work, scrap the idea of "approved" guilds and instead focus on a game mechanic that can be available to every guild, events that have to be completed by the guild and not just random members on their own. Develop a system that lets a guild work towards a goal.
For example:
1. The game GM's devise a series of quests that are designed for members of a guild... any guild.
2. All Guild Leaders can check an in-game posting, a bulletin board open to everyone say. This posting will give clues to how to start the quest line.
3. In order for the quest series to be completed, the guild leader will have to have signed up to it. After that, all members of the guild are allowed to pursue the quest. Only members of a guild actively working on a quest will ever see any of the quests.
4. Quests are accomplished in sequence by any member of the guild.
5. When all the quests are done or the guild has reached the last part of the quest line, an event is triggered by the Guild Leader to maybe spawn a boss mob, or any kind of encounter that you feel is appropriate for PS. Boss mobs have been done to death, other things can be done.
6. The rewards could be gold, items, or even titles for the members of the guild. Or perhaps a guild board on the forum or in-game that tells of all guilds achievements. Maybe the first one to complete the quest gets an in-game area dedicated to them, a plaque or something.
This would be fair and open to everyone, would remove any kind favoritism, and then the only thing you'd have to deal with is non-guild members saying there is extra stuff for guild members, but the quests should be designed to be accomplished by a group of people, and not a solo player.