Actually, we *do* have something that might suit what you want...

Have you tried the "alpha" and "boxed" features in the GUI? This refers to the 'A' and 'B' button at the top of the GUI near the "Thr" button. It's described a bit in this youtube video (give it a minute or two of preamble from this time point... sorry, seemed to make more sense that way...):
[youtu.be]

You can set the threshold value so that above it, results are *opaque* and *boxed*, and those outside of it are increasingly translucent. It is also described here:
[afni.nimh.nih.gov]
... with the drivable form of the AFNI GUI to make images.

We use this a lot to present statistical results. This seems exactly what you want, and the purpose it is there for.

--pt