[dragonwriter]

> Many libertarians (Milton Friedman among them) support a concept like this.

Most of the libertarian supporters (Milton Friedman among them) are dead, making them not particularly effective activists, though.

> It isn't a left/right thing.

It actually is, but a left-and-(right-libertarian) thing, not a left-vs-right thing.

> In fact, many liberals staunchly oppose UBI-type initiatives.

liberals (center-right) tend to, yes, progressives (center-left to left) tend to be in favor.