I wouldn't worry about anyone cutting the post or removing the sign from the top.
Those posts are a spade-base and usually just jabbed into the ground without a concrete anchor. It's likely that someone could just rock it a few times and yank it out.

Don't lock to a stairwell railing. Even locked to the outside of it, the fire department considers it a violation.
I'm a fan of the hipster high-lock when there's no rack available. Look for a fence and have at it.