Hi BJ,
from template options you cannot, but....Think out the box.
If offline page must be displayed ONLY for users that are not logged (guests) - just make that page default (home) menu item - public.
- And whole rest menu items will be visible only for logged users (Super User).
- Then using Custom CSS on that offline page - hide header and footer sections:
#sp-header, #sp-footer, #sp-bottom {display: none; visibility: hidden}
- In Joomla Admin > Global Configuration > Shared Sessions : Yes
That's it in a nutshell, but I think you caught the idea already.