Your problem is interesting indeed.
In addition to what others have suggested and what you've already tried, you
might consider clusters of lights. Four instruments next to each other focused
in the same place and dimmed to account for the brightness of a single
instrument will create a much-less-distinct shadow.
High sides should work very well for you also. Perhaps you can add special
emphasis on the near-side to compensate for your camera's spherical "eye".
Try fooling with different instruments. Some of the wide or medium-angle PAR
lenses create intense light that doesn't have a distinct shadow.
Light your shadows. If you're getting shadows on the floor or walls that
confuse your cameras, you could try compensating by
pointing a light on that section of the scenery to "fill" the shadows.
Similarly, you may be able to light-wash your floor in such a way that it
overpowers shadows, and then filter that light (maybe through color) out
of your overhead camera's view.
Footlights could work, but create a very funny visual effect. A
reverse-foot-light (a cyc light with no cyc - striplights accross the upstage
floor) will seperate your dancers from the background, and if focused
correctly, may not goof up your cameras. It's a funny visual effect also.