The ES Plugins Docs state that you can use -Epath.conf=/path/to/custom/config/dir.
From ES_HOME, I've tried the following;
bin/elasticsearch-plugin -Epath.conf=/etc/elasticsearch/custom-conf-dir list
bin/elasticsearch-plugin -Epath.conf=/etc/elasticsearch/custom-conf-dir/elaticsearch.yml list
But I always get the following error;
ERROR: E is not a recognized option
I've also tried with the old command line switch, -Des.path.conf=/path/to/custom/config/dir, but that produces a similar error;
ERROR: D is not a recognized option