If the read/write/access speed is going to be good enough then perhaps it might replace HDD and depend on SSD as another cache level. The SSD cache would need to be page-file-based (just like OS caches file pages in RAM). block-based caching would be horribly inefficient in this case.

This also got me thinking about a filesystem for the 'superman memory'. It could be an extremely simple logging filesystem without garbage collection. Since the storage is supposed to be enourmous this could work for more than just a an incremental backup.