Are the radial spokes in Saturn's rings reliably visible via ground-based telescopes
Bryan (2007) gives a number of reasons why O'Meara's discovery was largely discounted:


As you stated, the behavior was entirely inconsistent with Keplerian predictions of motion.
While O'Meara was able to reproduce his findings, no other independent observers could.
The detections were done entirely visually, rather than with numerical measurements.
There had been previous observations (i.e. in the 19th century), but in different places in the rings (the A and C rings, not the B ring). These have never been confirmed, and it is still believed that these were actually illusions.


Reliable observations from ground-based telescopes, especially those of amateurs, have arisen within the past decade or so. Since 2007, 67 "candidate" observations have been made of the spokes, many during the 2009 Saturnian equinox (keep in mind that the spokes may be a seasonal phenomenon). One additional factor that made observations difficult was that the spokes seemed to have vanished, even from space probes, in the years after the Voyager observations, making it impossible for ground-based telescopes to see them.