I've yet to find anyone else mention this, but after creating a new hallway in the Museum of Technology, I find I can stand right against the wall and see through to
the void, especially worse when turning in a corner. My walls are snapped/aligned correctly.

Do you mean you can see through the wall or see throught it at a joint or crack? If you can see through the object itself i believe this is a camera problem i have npticed on a few other games. if you stand right at the wall and rotate the camera a bit it can eventually go through the wall. Is this what you mean?

That's it exactly - although I haven't witnessed it anywhere else in the game, leading me to think it's some error I've made in the GECK.

Well from the details i have gathered so far i think it may be caused from the collision data somewhere, With my brothers muck around mod we test with i cant but even if i disable the wall i cannot even walk past it like there is a wall. I will PM some of the more experienced modders i have seen and get them to have a look into it for you and hopefully we can resolve this issue for you

Hmmm, well if I had to gander a guess here, since you are adding to a prexisting cell. I would guess that its a roombound issue. On the top toolbar click View then Show/Hide and check Roombounds (Or room markers or something I'm at school so I can't look at what it is exactly.) and it should show them in the geck. If it is cutting into your new hallway it will cause clipping issues/dissapearing chunks of map. You can either reshape it to fit around your room, move your new hallway elsewhere, or add room markers to your hallway to alleviate the problem. Look at the GECK wiki on optimization as it explains how room markers/bounds work in detail.

Ah yes, that was it cheers. My extension of the Halls of Today in the museum of Tech went beyond the original room bounds. Phew.