F5 Maintenance Page iRule produces ERR_CONNECTION_RESET in browser

I think the following might do what you're looking for; https://my.f5.com/manage/s/article/K14702105

You can add this to any existing iRule, or just add it as a separate iRule to any eligible Virtual Server. This also means you don't have to worry about updating anything when you notice outages happening/will be happening soon and no further automation needed. 

If you'd rather have a bit more elegant outage page, you can also replace the HTTP::respond command with an HTTP::redirect and send the user to a completely separate web server for a better page. 

Hope this helps.