First off, I think it's a bit unfair that you had your question up for all of 12 minutes before giving up and going right to FFG (if indeed you do that). The forums aren't always instantaneous, and for what it's worth, you got a good answer to your question quite quickly.
Having the book out in front of me, the idea of the OL placing monsters on the Exit to deter code guessing seems perfectly legal within the encounter. Whether you think it's cheap or bad sportsmanship is of course within your right, but remember this is a competitive game: the OL is actively trying to win. Why wouldn't they do what they thought was necessary? As a mostly OL player, I think this is pretty ingenious, and will have to give it a try upon our next run through Labyrinth of Ruin, if we take the Raythen route (my heroes like Scout characters so Serena seems to be the better choice so as to have some healing capability).
Your reasoning for why the encounter can be difficult is quite sound: it's early in the campaign. Heroes might not be as well equipped as they'd like to be. But for the OL, that's his/her time to shine until the heroes hit that point of the power curve where they can quite easily massacre anything put in front of the heroes. Yes, there's a lot of chance and luck involved, but there's just as much for the OL.
I'm a hero player in my current campaign, so here's how I would have approached Honor Among Thieves, encounter 1: Jain Fairwood blow her feat to fly at the open group on the Rocky Path. If the open group are large, such as merriods, plague worms (which you mentioned), golems, etc. try to at least wound something. Get Beast Master Syndrael as close as possible, and within another turn, a wounded monster could easily go down with her Skinning Knife. If the open group is a bunch of smaller guys (such as the witchers), Jain may well kill one off on her own. In this hypothetical game I still have two heroes actively looking for blue objective tokens, and the open group can't reinforce, so in 2-3 turns this OL strategy could very well not be so grand. Even if it takes, say, 4-5 turns to kill the open group, at least you have heroes exactly where they need to be to dive onto the exit and start guessing the lock.
Of course, your hero group probably looks nothing like this, but the point is you have options. I like to play recklessly so that supposed plan might not form to your play style, but the option was there, so I took it.
That was a lot of thinking for something I'd completely gloss over normally. In summation, I don't think your OL was being that unfair; the exact premise of this game is trying to outdo the opposing side, and your heroes got outdone. Better luck next time!