Here's the most recent insulating brick experiment.http://www.pizzamaking.com/forum/index.php/topic,19236.0.html
As you can read, he was able to hit high temps for the first hour or so, then the burner shut off. The high temps could easily have been the blocking action of the tile ceiling and not the insulating brick.
Even if insulating brick buys you a 180 deg. increase for an hour and a half, by the time the pre-heat is done, that won't give you enough time for more than 1 pie. There's also the question, with cordierite, as to whether or not a 180 deg. bump will be sufficient for Neapolitan undercrust char.
Member Malanga (Andrew), seemed to construct his brick fairly conscientiously and talked about the hole being small enough for a snug fit. I don't really see any way of improving on that.
It was just one test in one oven, though, so I'm not giving up on insulating brick, but the results were disappointing enough that, for the moment, I'm going back to the drawing board.
Cake strips are fundamentally just cloth, so, even wet, they wouldn't be suitable for use that close to the broiling element.
Ceramic fiber might work, but I have concerns about breathing/eating these kinds of fibers, so it would have to be enclosed. Due to the proximity to the broiler, I'm not sure I'd use aluminum.
The frozen towel trick has the best track record, but, I, too, would be concerned about the proximity of the element.
Dan, while a gentle mod will give you sufficient temps to give you good NY style undercrust browning in 4 or so minutes, because of the placement of your probe and the difficulty implementing a mod, I think you could make your life a lot easier with a 1/2" steel plate that will give you that browning at 550.
As you'll probably notice, I am talking about NY here. I can tell, by looking at your broiler elements that they will never produce Neapolitan leoparding. At least, not without really extreme potential oven jeopardizing temps- and even then, I think it's a long shot.
At the end of the day, either your broiler has what it takes to do Neapolitan or it doesn't, and, if it doesn't, imo, it's better to embrace NY- and work towards achieving NP on another piece of equipment, such as an LBE or a WFO.