I think you have to ask yourself first, "what am I trying to do"?
There is one company that uses a compression driver into a horn that is aimed down, there being a complementary curve below, with the exit being 360 degrees rotated about the vertical axis.
You might want to look around for that?
Not quite "omni directional" but definitely doughnut shaped certainly.
At bass freqs most woofers are omnidirectional or at least hemispheric.
As the frequency goes up for any vibrating surface the wavelength gets shorter. As the wavelength gets shorter the surface radiating the sound begins to emit the sound in more and more of a direction perpendicular to the vibrating mode. The critical point is when the wavelength gets around 1/4 wavelength distance compared to a dimension of the vibrating object.
So one answer is to use smaller size surfaces to make sound below 2kHz. Of course the problem with that is merely max output level.
That can be solved by a multi driver line source - which then resolves to a longish ribbon driver of narrow dimension.
You can also look at the Walsh driver solution as well.
Unfortunately there is nothing that is inverse of a horn unless you consider some spherical or egg shaped object with a driver mounted on the surface - but it will have no "wavguide" properties per se.
You could
scale up the above described "doughnut" horn...
_-_-bear
