Would have to be hotels.
And the government would have to buy them first
In the late 80's and early 90's NYC kept detained immigrants in hotels that the gov't owned. I think they had been condemned or taken for back taxes, etc. Problem was most of these former fleabag hotels were in the heart of Manhattan and had many exits. So the staff simply starting chaining people to the radiators. When this came out it was a scandal (of course in violation of fire codes, but also inhumane). And they stopped.
My point is that if they could do it with overflow immigration detainees, they could (probably) do it for homeless people who are allowed to roam around. I've been away too long to know anything much about the building stock in NYC, but I imagine there are buildings that could be condemned or taken for back taxes, or that the city already owns and just need fixing up. Former hotels are good, but it's also easy to put up interior walls and make a communal kitchen on each floor of a building.
I mean were talking a city where some folks make a hundreds of millions a year.
And a city that (normally) relies a fair amount on tourism.