When using HDFS, the Overseer should have the ability to reassign the cores from failed nodes to running nodes.
Given that the index and transaction logs are in hdfs, it's simple for surviving hardware to take over serving cores for failed hardware.
There are some tricky issues around having the Overseer handle this for you, but seems a simple first pass is not too difficult.
This will add another alternative to replicating both with hdfs and solr.
It shouldn't be specific to hdfs, and would be an option for any shared file system Solr supports.