I don't have any particular problems with what you're saying, although it feels like a lot of the abuse around camping and doing work will be offset by the problems of stigma associated with being in one of the centers. That is, there's no need to police people to make sure that they aren't doing outside work or reading, when being in the center itself is not a particularly fun -- people who can afford to not be in the center will most likely choose to spend their time elsewhere.
That may not sound particularly humanitarian, but keep in mind that this also benefits the "smelly homeless people" likely in ways that the current systems (what good is a minimum wage when nobody will hire you) or UBI (what's your mailing address, sir, where we can send the check, or where's your photo ID?) will not.
Addressing all the addressable avenues for abuse is also going to be a grim looking situation where you're going to be policing people sitting there to make sure they're not sending e-mails from their phones or having conversations with each other or reading books. Lots of peoples' jobs involve studying or having meetings.