Hi Jon,
The Scheduler does not expose a configuration option that would allow you to force it displaying the midnight line. Nevertheless, you could apply a CSS rule in order to make the border of the respective cell more visible. Here you could find a small sample demonstrating that. You will notice that the CSS used in this case is the following:
Note, that you will need to adjust the above rule according to your specific scenario.
Regards,
Veselin Tsvetanov
Progress Telerik