What enforcement is needed? Either you opt in to accepting the UBI currency, or you don't. If there's an enforcer involved then you've got government UBI.
As for issuing the tokens, there are a variety of UBI systems that handle that without a centralized authority. CirclesUBI and Idena come to mind. Admittedly some work is needed here, but the technical barriers are not the hard part.
The hard part is establishing the collective political will to build a culture around accepting them, otherwise it's just meaningless numbers.
That sets up a problematic power dynamic. Whoever is doing the giving has power over whenever is doing the receiving.
And you'd want it as an alternative for cases where the other money is questionably legitimate. Like if the people who control its supply have stopped acting in the best interest of the people who use it.
If anything, that's an argument in favor of UBI - it reduces this power by making the giving unconditional. If every citizen is automatically entitled to their equal share, what power dynamic is there, exactly?
If some centralized entity is responsible for disseminating the income, they can threaten to stop, or they can threaten to cut people off.
So I agree, universal solves the problem, but you're not going to get universal from a bureaucracy. You've got to bake it into the design, similar to how backed-by-gold is baked into the design of the money we're currently using. It needs to be legitimate because it's universal, rather than being legitimate because you got it from somebody who has a lot of guns and has promised to behave themselves.
I'm very pro-UBI, I just don't think that saying pretty please to the government is the way to get it.
The government is not some random entity, though. If the bureaucrats don't actually follow the laws written for them, they can be fired (through courts if needed). If the legislators threaten to repeal the laws, they can be voted out.
As for issuing the tokens, there are a variety of UBI systems that handle that without a centralized authority. CirclesUBI and Idena come to mind. Admittedly some work is needed here, but the technical barriers are not the hard part.
The hard part is establishing the collective political will to build a culture around accepting them, otherwise it's just meaningless numbers.