The Clan Wolf-Steiner SA allows you, at the start of your Order Stage, to allocate Arm P to one of your ballistic units. The exact wording is as follows:
1/450: At the beginning of your order stage, choose a friendly unit with the ballistic range type on one or more damage values: It has Armor Piercing on that damage value or values and cannot use any other damage special equipment until the end of the turn.
So, I'm thinking that this would give the Warwolves Arm P (replacing any Streaks, for example, for the turn this was used). That's fine and dandy and could be pretty useful and a threat to Hammerfall. Actullay, recruiting Hammerfall to CW and giving him Arm for 40 points is cheaper than paying 50 points for the CEC.
However, what about the Steiner Towed Thunder Launchers?
Could I assign a Launcher with the Arm P SA on the turn it places its POG and then re-assign the SA to that unit at the start of my next order phase, just before the POGs resolve, thus landing a pair of Arm P POGs?
Even better, could I simply assign the SA to the Launcher at the start of my order phase, just before I roll to resolve the POGs?
No -- artillery resolution happens in the command phase. You can't give a unit armor piercing until the order phase. Even if you gave it to the arty the turn before, it loses it at the end of turn. Sorry, no cheese to see here.