I found this! http://volumetricshadows.files.wordpress.com/2011/06/fourier-opacity-mapping.pdf
I have no idea how it works, but it looks completely awesome. It doesn't require any particle sorting when rendering the occlusion map, and it also supports prefiltering, which (if I got this right) should mean that we can also use mipmaps and anisotropic filtering. I can see myself using this for translucent stuff and exponential volumetric shadow maps for opaque stuff. Both can be filtered in hardware, so they shouldn't suffer from aliasing even when viewed at step angles thanks to anisotropic filtering or when viewed from far away thanks to mipmaps.
EVSMs need 32-bit float textures, which I believe are supported on OGL 2 hardware through extensions, but I'm pretty sure that hardware filtering from such textures is not, which pretty much kills any advantage of having them. Those Fourier Opacity Maps work with 16-bit floats though and should be easily implemented even with only OGL 2.
*Added to my to-implement list.*