Hi guys, just came across this thread, because I just had the exact same problem.
So far my dirty workaround is by using a negative mask of cryptomatte.
Since the volume doesnt show up on the cryptomatte pass but everything else does, what Im doing right now is basically select every material on cryptomatte and then invert that mask, which gives me basically the volume.
Of course this only works well if you dont have a transparent background, only if basically all the volume has surfaces on the back.
Hope this idea helps out, I know its messy, but like I said , just a dirty workaround