A summary of what i know so far.
Stilwater in the first mission is sitting on top of the open world boundary, for some reason. It was probably designed and placed at some distance early in development, before the boundaries were decided on. The mission script loads the "stilwater" zone, and there are a couple of bank-related zones as well.
We can load Stilwater, but if you try to get too close to the bank, you're pushed back by the map boundary. You can see the very top of the bank, but the rest of it either isn't visible unless you're on the other side, or is entirely not loaded. Maybe the interior of the bank exists in the m01 files, but if that were possible, you'd think that they'd put the entirety of Stilwater in it, and i don't think any other missions work that way. No, i suspect that it's just not visible from the outside. It's hard to tell when you can't get close, though.
When you're in the mission, the boundary is not enforced. I don't know why. There's nothing in the mission script that toggles it, so it may be hard-coded in the engine, or it may be something that's changed by the game loading the zone information that's inside the mission files. Surely the engine enforces the boundary during other missions, especially those where you have flying vehicles? So if it's hard-coded, it's coded to do it only during mission 1.
I wonder whether it's possible to swap a couple of the missions around, and what would happen if we did. Shouldn't be too hard, just swapping filenames... ah, but the asm file expects them to contain certain files. Well, maybe this is not so easy. I guess i need to expand the asm file and do a little xml surgery.
By playing with the city lua file, i can have it put me into the wrong location after starting the mission, which lets me return to Steelport on foot, but it would take some modding effort to make it let you enter a vehicle (m01 prevents you from entering vehicles, normally). Probably not worth it, but hey, you could then presumably fly around out of bounds all you wanted. Hoo rah. Given the way this game handles most unexpected events, it would probably just crash after a while.
You can teleport yourself to the area beyond the boundary, but it's still enforced: the game takes control away from you and has your character (very clumsily) attempt to run back toward Steelport until you're within bounds.