At the top and bottom of the hourglass, there is a hidden opening, an invisible trapdoor opened by the application of a magnetic field. The pedestal applies this field at the right moment, gaining access to the hourglass to mess with nature itself.
The pedestal contains a further reservoir of sand and a special pump.
With the sand in motion, a glass capillary tube is extended vertically into the hourglass, reaching up through the flow of sand all the way through the constriction and into the upper chamber.
Sand is pumped up through the capillary tube and into the upper chamber. As such, the falling sand never stops.
In the following two figures, you can slide left and right to compare renders with and without sand, to reveal the hidden mechanism.
While operating, the tube is hidden behind the continuous curtain of sand.
If motion is detected, the capillary quickly retracts and the secret opening snaps shut, so the hourglass can be inspected without revealing any of the magic. To protect the delicate glass capillary, perhaps a bayonet fitting between the pedestal and the hourglass would be required. But that's just an engineering detail.
This infinite fountain of sand is an art piece, a statement, a magical contraption, maintaining the illusion of endless time, but there's also scope for further features. With software control over the mechanism, the hourglass could be configured to last specific, exact durations, synchronised to a quartz crystal. If you wanted it to last for exactly four hours, ten minutes and thirty seconds, it's only a (hidden) configuration away.
How quiet could the mechanism be? For the illusion to hold, the entire thing would have to be near-silent. Getting the pump quiet and efficient may represent a significant technical challenge.
How fast would the sand exit the capillary? It's important it doesn't disrupt the cone of sand in the upper chamber too significantly. It may be that some kind of diffusor is needed on the end of the tube.
As ambitious as this artwork is, as an intermediate step, I concede we could build a partial version, where the hourglass is permanently attached to the pedestal, and the capillary tube doesn't retract. Once the proof-of-concept is demonstrated, we can proceed to the real thing.