Just to follow up on this with another thought - my suggestion about the "cutout overlay" is sort of a kludge. I would still like to see it, but perhaps it is not the best solution for displaying activation that lies just beneath the surface. The problem with "showthrough" is that there is no distinction between blobs that are just beneath the surface and those that are in the middle or other side of the brain.
Another solution that could be even better is some kind of adjustable "depth weighting" function, that allows activity within a certain depth from the surface to be displayed. This feature is used in several other rendering programs such as MRIcro, MRI3dx, and SPM. So my current needs can be met by using one of those, but since I'm so far a lifelong AFNI partisan, I'd like to urge the further betterment of the volume renderer.
-Jed