I have noticed this issue have been lying around for a year, seems like nobody is bother to use JOIN in solrcloud, so I decide to tackle this myself.
A small patch has been uploaded here to allow fromIndex to specify a collection under cloud environment. Currently, it works if the fromIndex collection is a single shard having at least 1 replica on each node. I am planning to support multi-shard collection but not really sure how to do it given I am not that familiar with the internal mechanics of solr.
Even if we support multishard, given the current implementation of JoinQParser, I think we can only support it when a collection with at least 1 replica of all shards physically residing on every node. We need all local IndexSearcher. If we need full solrcloud join support, I think we need to revamp JoinQParser or make something on a higher level to gather terms collection from remote shards on StandardRequestHandler.
By the way, I noticed we haven't use JoinUtil in LUCENE, is there a reason to not use it, their implementation seems to be more cleaner than the one in SOLR right now, I have no idea how JoinQParser works, especally the getdocset stage.