"concerns about legal vulnerabilities" 
Those are valid concerns, hence the failure to approve an island-wide sit-lie ban.  Judges are not likely to allow zero options for homeless people.  Judges are inclined to protect "life, liberty, and the pursuit of happiness" unless it infringes on the rights of other people. 

Create and define "zones of significant common public economic and recreational benefit" where overnight camping is prohibited without a permit.  These zones could be defined in many areas throughout the island, but would be more common near Waikiki and downtown.  Like zoning, but for public lands.

But realize that the squalid conditions (similar to what was under the airport viaduct) will spring up somewhere on the island.