Your proposition is to make players totally invisible. I guess it could be done based on 2 skills like you mentioned.
But first they need to implement the crouched position, then sneak rise while crouching.
At skill 60 you could knock un-counscious a target facing the other way if he stays immobile for 2 seconds.
To make it interesting you could add back stab at skill 90, and pick-pocketing at 100.
to level-up the patrol skill travelling at night with a torch equipped or travelling on your claim during JH should work. At skill 90 I would add something like the ability to consume a rope to use a lasso on a character at war with you to capture a slave at lower alignment cost. at skill 100 you could use it on horseback.
I feel like it should be hard to succeed unless the guy is over weight. By forcing the player to bring a walking slave at the outpost before allowing release (a bag with the slave's possession would drop in front of the outpost) would make it less OP.
The capture shouldn't be instantaneous a mini game should appear giving the future slave a chance to break free, the mini game should be easy if the slave is carrying a knife very hard if he's got no blade equipped and one arm is broken.