hmmm, make a teleporter that looks like stairs or elevator? then it teleports you to the *second level*?
Yeah, that's the easy way to do it. What abour air pressure? You could handle it in a simple way but it wouldn't necessarily be a good way, and it would be a noticeable slowdown.
My biggest concern is the gameplay implications. If you can't see someone on the next Z-level up, that has HUGE griefing potential. And as much as I want to have aliens in the ceiling, how would you represent a tracking device saying "they're in that square, but up"? What if they were four Z-levels up? Also, wires and things, how do you depict wires and pipes going up? I guess you could put a little red up-arrow, but it's starting to annoy me already.
Not being able to view things above and below you, even when you have LOS, is a killer. Especially when you'd probably be able to hear them. Way too easy for one guy to go to some kind of big habitation dome, build a ladder in the middle, and snipe people who don't think to look up. Too easy to hide things 'up'. Etc...
If you think we could use adventurer-mode style interface with a little box showing up and down, remember how very small ss13's viewscreen really is.
I can think of one way that Z-levels would work: If the only interfaces between them were more like ramps. IE, a spiral staircase. You NEVER run into a border, you just act like you have a Portal or something that you can see through and walk through without even knowing you went through it. All you know is now you're on floor 1, and you go around this loop and now you're on floor 2, without any obvious transitions.