Instead of using open you can just run the binary inside Slack.app directory:
SLACK_DEVELOPER_MENU=true /Applications/Slack.app/Contents/MacOS/Slack
72 karma · joined January 4, 2015
SLACK_DEVELOPER_MENU=true /Applications/Slack.app/Contents/MacOS/Slack
SLACK_DEVELOPER_MENU=true /Applications/Slack.app/Contents/MacOS/Slack