At the time I implemented the versioning using separate trees on top of the Radix tree. Today I would augment the tree with t he versioned nodes, and would use a Concurrent Adaptive Radix Tree for the base tree. This would allow high concurrency. Then at the base of the tree I would store a temporal link to each tree modification, so that you could access the changes in time order!

Very, very interested in chatting further about this!