Follow-up: Had a bee in my bonnet, and think I have some ideas as how this might need to work.
First, the "prebate" is the right idea. Every adult citizen or permanent resident gets it, no need to do anything special. Direct to your bank account, every month. Adjust tax brackets as appropriate.
Enforcement for the "must work" provision would run through the tax system as well.
You need to be able to show yourself as working for an organization which has a tax ID, which should include nonprofits, churches, government agencies, co-ops, startups, private companies, global megacorps, all of them. That organization also (already) has to report you as an employee, so no major change there.
And while groups of people could form co-ops and nonprofits that claim to "employ" each other, that's probably fine. If you're willing to go through all the work to handle the tax filings and related compliance, and do so without committing fraud, I don't see a problem there.
Basically, hippie communes are fine as long as you do all the relevant tax paperwork and meet the legal requirements for being a non-profit/startup/what-have-you. To me, that sounds like a lot of work to do nothing, and probably just easier to work for the city government, cleaning up trash in the park.
Allow a decent amount of time to change jobs on an annual basis -- maybe a month?
Work requirements are waived if you are disabled, elderly, or a student (university, trade school, etc.), although the latter comes with a hard time limit (this is similar to how things work in Germany right now, by the way).
You can also elect to stop or restart UBI payments at any time, and the above work obligations are suspended when not receiving UBI.
Paying for this is a question -- would need to run the numbers on corporate tax receipts, but I suspect a combination of corporate income taxes, tariffs, and the savings from no longer needing to run a large chunk of economic aid programs (social security, etc.) would cover it. Not sure, though.
VATs tend to be regressive, so I'd be pretty strongly opposed to one of those. We'd also need to get rid of minimum wage laws (covered by the UBI), and public-sector unions likely also need to go into the fire -- they'd fight tooth-and-nail against UBI-paid "volunteers" doing work for the government, because those people wouldn't be paying union dues.
Modulo the numbers working out, and that there are plenty of special-interest groups that would rather maintain the status quo, I think this would probably work...