Thanks! I created a fractal heightmap and converted into a polyhedron for OpenSCAD, then differenced the polyhedron out of the barrier again in OpenSCAD.
The fractal heightmaps have turned out to be super useful in terms of creating damaged things, but if I were to do it over again, I would probably pull the pieces into Blender and do the differencing there. It's a lot faster than OpenSCAD, and it is much easier to tweak the position of the heightmap polyhedron that way.
As an aside, that's basically how I've been making the terrain for the bases on the wreck models, etc. -- I use a script to create an outline for the base and a heightmap from OpenSCAD to form the base (intersecting the two), but I pull them into Blender so I can tweak the heightmap beforehand (e.g., to get the edges of the base close to 1mm).
I don't really advertise it (until now, apparently), but most of my models and scripts are on Github.