I was just thinking about how to best implement this, and then I find a thread making the exact same suggestion... Oh well, I suppose the idea can't be particularly unique.
I can see a number of benefits to this beyond turrets. When planning to make this suggestion myself, the primary application I had in mind was for fighters. They're generally kind of thin, and needing to dedicate the entire outer layer of blocks to hull, in order not to look hideous, is really damn limiting. They could also be used for interior walls.
The issue with implementation I was thinking about is rotation.
The most simple way to deal with it would be to rotate it like any other block, facing the plate towards any of the six sides. This would work for flat surfaces, but not for any kind of inwards corner. That would be annoying for fighters, bad for turrets and outright horrible for interiors.
Optimally, each side would be toggled independently. Unfortunately, that would require six bits for six sides, and the game won't go any higher than five. (Four rotation bits plus the activation bit.) I suppose you could cannibalize an hp bit to get the job done, but that would still leave an awful lot of rotations to cycle through in build mode.
What I think I've settled on is having no rotation whatsoever. Instead, panel "blocks" would adapt to their surroundings. All sides of the block would be transparent by default, and only render when they're on top of a solid face. So, placed in the air, a white panel block would be invisible. Placed on the floor, the top of the block it's placed on would turn white. Placed on the wall, the side of the block it touches would turn white. Placed in a corner, both/all three of the surfaces it touches would turn white, etc. This covers nearly all uses, except some that may arise when using solid blocks for interior detailing. Such cases would easily be solved by using normal hull in those particular spots.
There's also the matter of thickness and hitbox. To make it as simple as possible, I suggest a thickness of 0,000. This would eliminate any issues with surface-to-surface sliding rails and make outwards corners not horrible.
The hitbox should be the individual, rendered panels. Of course, since they would be two-dimensional it would be more of a hit-surface than a hitbox.
