I have never written a path tracer, so take my response with a grain of salt. I don't see why you couldn't have a child node in your BVH be another BVH with a transform associated with it. In fact, that makes the most sense to me and I would venture to guess that production systems do exactly that. What complicates things, like you said, is if you are morphing geometry or otherwise changing/rebuilding the BVH for a dense object with millions of triangles. You should give it a shot and let us know how it goes!

I have never written a path tracer, so take my response with a grain of salt. I don't see why you couldn't have a child node in your BVH be another BVH with a transform associated with it. In fact, that makes the most sense to me and I would venture to guess that production systems do exactly that. What complicates things, like you said, is if you are morphing geometry or otherwise changing rebuilding the BVH for each object. You should give it a shot and let us know how it goes!