I assume you want to use standalone ES instances in order to reduce the network traffic and ensure that indexing is done locally, is this correct?
If so, what you can do, at least as long as you do not need to update your records at a later stage, is to set up all your indexing nodes in a cluster and configure the number of shards so that they have one primary shard each and no replicas. Then index a number of records so that at least one record has been written to each shard. Retrieve the id of one record from each shard and use this as a routing key for all records you want to index into that particular shard.
This will give you a single index with a number of shards, while it still allows all documents to be indexed locally only (as long as you provide the correct routing keys). It is not elegant, but I think it should work.