Fact or fiction? Active prop balancing
 

Has anyone tried something like this or is it the next snake oil?

Lycoming Balancer

It will just be a fluid (or fluid like mechanism - eg lead shot, silica beads, etc) inside an annulus. Trucks use a similar technique for tyres. Washing machines use a water filled annulus to help balance. It should work, but really its a band-aid. Do the balance properly!

Akro is correct, the tyre balancing rings have been around for several decades, and they do work. There are many styles and positions of fitment, and some are much better than others.
The best ones consist of a tubular ring containing heavy-viscosity fluid and a few steel balls, that are nearly the diameter of the rim, and which fit into the rim flange.
The problem I see with the balancer shown, is that it will have some benefit as regards crankshaft and other heavy rotational engine components are concerned - but because its diameter is considerably smaller than the prop, its balancing effect on the prop would be minimal.
The balancer needs to be nearly the diameter of the prop to be totally effective on prop imbalance.