Well, the source code is available: http://www.lesbird.com/CAPSTONE/default.htm
But that still doesn't mean a port of it is likely. The code won't compile on modern operating systems, so someone would have to rewrite it first.
But there is some hope, as Witchaven seems to use a version of the build engine (the one used in Duke Nukem 3D and Shadow Warrior). So one with enough time could merge this code with modern version of build.