Im tossing this out there. I don't think that it is a certain thing that actually unlocks the FSS. I think that on every thing you do you have a chance to unlock it. Say killing a pickett gives you say a .00045% of opening the slot. By switching professions you are trying different things that have different levels of chance to open the slot including the training.
So to make this clear think of it as a random die roll every time you get training, kill something, walk in to a town, or even craft something. Maybe these are cumulative, so the chance of unlocking the slot increases with the number of professions mastered.( going from a 0.00045% after one profession to a 0.0065% after 4 professions.)
This would insure that everyone stumbled upon there FSS like they have been, but also made it possible for a casual gamer to "luck out" and get it too. Also hardcore gamers would be reward since the roled more often than the casual gamer.
If this holds true then doing anything in the game would allow the slot to open so badges, grouping, PvP, PVE, getting a new skill, getting a new rank, declaring residence would all have a chance of unlocking it. The more you do teh more chances.
SO what do you guys think?
Because thats my story and I'm sticking to it.