Polarizing filters are probably not useful, since most bulbous lenses are very wide angles. Polarized light changes in strength at 90 degree intervals to the sun, so if your lens is wide enough to see a large enough portion of the scene, the effect of the polarizer will very noticeably change from one side of the photo to the other. So you'll get bands in the sky, reflections in one part of the photo but not another, etc.
Graduated neutral density filters would be useful if it is a rectilinear lens, but not if it's a fisheye. Usually you don't place the horizon at exactly halfway in the frame, so the horizon in a fisheye is likely to be curved, and won't match up with the gradient in a graduated neutral density filter. Maybe for a rectilinear tilt shift, but ehhh.
Plain neutral density filters would be perfectly useful for both fisheyes and rectilinear lenses, regardless of their width.
Also, you have to keep in mind that very wide lenses are also more likely to get vignetting from the sides of even a single filter. Especially fisheyes that see up to or even beyond 180 degrees -- a single filter housing could vignette like... 5-10% of your image potentially. And potentially much more if you have some crazy hacked together contraption in order to fit the filter on.