How can messages be scripted whenever I open a specific application?
Run EDITOR=nano crontab -e and add a line like this:

* * * * * pgrep -xq Safari &amp;&amp; terminal-notifier -title 'Click to quit Safari and open Chrome' -message '' -group cronsafari -activate com.google.Chrome -execute 'osascript -e "quit app \"Safari\""' &gt; /dev/null

The command is run about once per minute, and the notifications are shown repeatedly as long as there is a process named Safari.

You can install terminal-notifier with sudo gem install terminal-notifier.