It's possible to transclude a system message using the visual editor by typing, for example, Int:Randompage in the template dialog. However, it's not very user-friendly.
Ideally, we would:
- Somehow distinguish system messages from regular templates and provide a link to the help page (e.g. https://en.wikipedia.org/wiki/Help:System_message)
- Provide the lang=qqx message description in place of the template data description.
- Make it possible to tranclude using the more common MediaWiki namespace (for example, MediaWiki:Randompage). Trying to add a message this way currently fails the template dialog's input validation.