Going to have to reply to myself on this one, because I've run into an absolutely INFURIATING bug now related to my original post.
Because now the invisible walls will work in the water I put them in...on the PREVIEW maps only. After publishing? People are telling me there's no wall there. Went to inspect myself using my main character, and yup, in the spots where the walls were working perfectly well in the preview maps, they were completely absent from the final product. Big 'ol 5k tall walls, placed -30 into the ground for good measure. Not there, but only if the walls are placed in water, looks like.
And here I was about to say "my first test maps work fine, on to do something serious". Now I kind of don't want to do anything else with the Foundry, if there's such bugs that mean no matter how much you test them in the previews, they could possibly not work at all as intended in the actual published version.
EDIT: You know, I should have probably put my published mission title in here after I actually published it, so that people could see what I'm talking about.
My test mission is called "A Room Filled With Crazy", by milesredria.
The problem shows up especially on my third map in the mission. You go up to a pond, interact with an object, a dialog fires off, and if you get a negative response to the dialog, you'd get jumped.
The way I have it set up is the invisible wall cage (made of 4 walls, and yes, there's no gaps) are to show IMMEDIATELY when that map starts, and the NPCs are added/warp in when you interact with a trigger nearby. "Fail" the dialog, and the cage drops.
And it does still work in the preview tool....but when you go and try it with the published version, nope. Walls in the water are gone completely. So the enemies warp in, and you just plain get jumped in the middle of the dialog instead.