You can create screens to be used as "transition screens" in the workflow. Greenhopper will pop these up if people transition through them. You can't use them (as far as I can make out) in the GH "simplified workflow" though.
See https://marketplace.atlassian.com/plugins/com.burningcode.jira.issue.customfields.impl.jira-watcher-field in general, and maybe jump to https://confluence.atlassian.com/display/JIRA/Configuring+Workflow#ConfiguringWorkflow-Usingascreenwithatransition for the specific screen stuff
...PermissionsStartOnly=true User=www-data Group=www-data ExecStart=/opt/jira/bin/startup.sh ExecStop=/opt/jira/bin/shutdown.sh TimeoutStartSec=120 TimeoutStopSec=600 PrivateTmp=true [Install] WantedBy...
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