Update: Found a surprisingly effective hack based on the "use local gradient to look up environment SH lighting" approach

Hack: just use a really, really blurry version of the local gradient to sample the environment SH

Softens the lighting so much it almost starts to look like scattering. Akin to the old wrapped-diffuse trick but more extreme

In this case I'm not even "properly" blurring the gradient, just doing finite-difference with a hilariously huge distance

here's varying the sample radius between 1 voxel and 30

the result is never Correct but it looks increasingly Good as it gets softer

This has a major shortcoming: It has nothing to do with transmission. Like, at all. It's picking up blurry *reflected* light from the probe

so like, when viewed from below, we see green grass reflecting OFF the bottom of the cloud, instead of seeing blue sky filtered THROUGH it

but hey if your environment doesn't have a ton of color variation, and you march a direct light to pick out all the details, it makes a really surprisingly decent approximation of inscattering

not "PBR" by any means but it responds predictably to differing environments