Okay, I just managed to do it, and for those interested in how to achieve that: you have to sneak your way in right from the start. Go through the West gate (need at least one char with Stealth above 10), and then sneak along the left wall staying as unnoticed as you can. I still needed to kill a few Radcerans, but I kept the numbers low. So now I was able to get a peaceful solution.
PS: Kudos to the RPGCodex, who pointed me towards the right direction again.
JoniOdin's post in The Iron Flail: peaceful solution? was marked as the answer