There are a couple of places you can set maintenance for a monitor immediately (as good as now).
Goto the monitor list page. Hover on the hamburger icon for which you want to mark maintenance -> the context down menu has a section to schedule the maintenance. You can set the maintenance immediately.
You can also do the same after click on the monitor you want to mark. In the details page of the monitor. There is a setting for schedule a maintenance. This will do the same functionality.
Seconds granularity is not possible now.
I hope this clarifies.
Product Manager, Site24x7