Here is an example of an anti-homelessness public/private partnership with extensive services, many provided by NGOs like you envision. It is only targeted to homeless veterans, so hasn't addressed the needs of many people, but has made a huge difference in one community. Perhaps it's a model that could be expanded widely. Note that the starting point was the rehabbing of a defunct motel. This suggests to me that it could be adapted to all kinds of empty buildings, as we are seeing changes in cities in the wake of the pandemic. https://www.columbiaha.com/housing/patriot-place-apartments