My office is a former slab-on-grade warehouse that was converted into offices, laboratories, and manufacturing halls.

They installed eight multi-stall bathrooms where once there was a single room in the corner with one toilet through the revolutionary act of running PEX and digging a trench in the slab for sewage.

It is trivial to do the same in a high-rise. If anything it will be easier because instead of digging a trench you can punch holes in the floor and route plumbing and ventilation through the plenum (or whatever is up there).

Water supply and power isn't a problem because here's a secret: commercial spaces almost always use more power than residential, especially in the extremely esoteric and rare cases where workers have computers and copiers. Hell all of these buildings have phat 3-phase service. A dude watching Netflix and using his microwave in a space that once housed a dozen workers, half of which had space heaters under their desks because the HVAC was set to 70 in the summer will be a break for the transformer.

It’s not even just the obvious plumbing providing hot and cold water, and the drain pipes carrying away waste water. You also need vent stacks for all the drains. You also need to put in a whole lot of hot water heaters and the associated electrical work to handle their demands.

Frame additional walls within the room that include additional plumbing lines, branching from existing lines as needed. Granted, you lose space but at least you'll have a fully compliant residence, no?

It's not whether or not it's possible, it's the cost of doing it.

Can you run plumbing through the ceiling of the floor below? I'd think there's plenty of free space there but it would mean you have access to your neighbor's pipes.

That's how it's done now.

I once had a clogged drain in my apartment. When it was fixed, I was politely asked not to run the water until they could inspect the ceiling in the unit below. They wanted to make sure that it wasn't going to leak on my neighbor below.

General building/fire code but even be a larger issue. I suspect you could probably solve the plumbing side of things but meeting building code might be the larger challenge.

I've seen plenty of "apartments" (really just a room) in NYC on Tiktok that don't have proper restrooms (though they do often have a small sink)

I think they can use/invent something like dirty water towers in NYC. use that for whatever the NYC use it for. Water there really proven dirty so it doesn't matter.