Quote from Doskei
It does suggest a new challenge, though: since this needs to be vertically repeatable, the only way to "open" and "close" the snowman windows will be to use a piston above and below the entire room that acts on a stack at a time. That means I now need a block which can be looked through but which will still push on blocks behind it. That might be just as difficult as finding a block that can be seen through but blocks arrows. :/
There may be no need to find such a block. Instead of using pistons to shift the blocks and make your golem visible, perhaps briefly toggling a set of doors would work better. Just open for a moment and re-shut. In fact, at the range they would all be within, a single tick of negative pulse would be more than enough to toggle it and aggro all surrounding mobs. This does represent the possibility of releasing the golem; this can be circumvented by simply lowering the golem by a block so that only his head is visible when the doors open, and he would have to jump within that single tick in order to escape (which is nigh impossible).
If you have trouble with it, I would gladly wire something up that could do this.
Also, on a side note: if you made a vertical delay so that each floor would open at the same time, you could maximize flow so that mobs were constantly spawning. Also, this design is killer; I applaud you, fine sir.