Perhaps every 128 x 128 block is a separate module, and they are moved and stacked like in the "Cube" films. Like, there's a plain wall block, a box dropper block, ball launcher / catcher blocks, the works. Once in place, each block can extend its wall / floor / ceiling section individually. So then the metal grating is behind the fully retracted position of each block and it wouldn't be in the way.
I think the Enrichment Center actually has a huge ammount of test chambers, much more than the 19 we see in the game, and the pistons are merely to adjust difficulty levels if the order in which a test subject has to go through them, changes. I reckon GLaDOS can easily get an elevator to go to a different test chamber. You remember the elevator shaft you have to jump into in the BTS part? It proves the elevator shafts have multiple levels.
