I never use the tavern hook, typically you need an NPC sponsor for a mission though. The Xathoqquan priests would be a good one. A corrupt business man who owns a factory or a warehouse, the mercenary guilds of the watch, a rather sinister magician, are all others. I tend to work out what kind of adventures im going to prepare then decide how the players are going to get the mission.

As far as rail roading is concerned, you have two real options in any rpg. Before the campaign starts talk to the players about the kind of adventure or story they want, then prepare material around the results of that discussion. If they agree they want to be mercenaries employed by the Cult of Yug around the dark alleys of Khromarium then you prepare that and they actually do that, rather than hopping on a boat for Zangerios straight away. The players do get choices on the details through the decisions they make in game but the overal idea of the game is decided.
The second option is to go pure improvised sandbox and let the players do exactly what they want. This requires you to improvise most of your material but allows total freedom to the players. Theres no hint of rail roading, but usually there is no predesigned detail in any of the adventures. This option, in my experience, can often lead to infighting in the players too as they take their freedom for granted. Second, in adventure fiction, the protagonist rarely has absolute freedom.

And since it's Hyperborea, whatever the set-up is, it's fun to tie it to uniquely Hyperborean things--the calendar, the festivals, and celestial cycle, and so on.

I have seriously started adventures with "You are crawling through the desert with 1 hp, no water, and riddled with arrows, having been ambushed and left for dead" and "You were sheltering in a cave and the entrance collapsed during the night, trapping you" and even "You wake up nailed in a coffin and buried alive".  That instantly raises the stakes light years beyond "you're sitting around have a mug at the local tavern and some guy offers you a job".  I wouldn't start every adventure that way, but as an occasional tool it really gets the attention of players.  I don't consider it a railroad because I just give them the starting point - where they go from there is up to them.

I've done a similar thing in my campaign. It wasn't an instant action scenario, but an event occurred external to the PCs - how (or whether) they responded was entirely up to them, but it was weird enough that they wanted to.

My experience with AS&SH has been that the adventures tend to write themselves. If you can capture the mood of the gazetteer you're good to go.