I think it would be helpful to be able to define custom global values that are accessible in the Administration section of the admin panel. Specifically the boolean value "PublicSiteOffline" comes to mind. using the example given here: http://www.sitefinity.com/support/forums/support-forum-thread/b1043S-bgdtaa.aspx
I was able to make a module that redirects external users to an offline message, and expanded it with a boolean value to determine if it should be active or not.
unfortunately changing the value causes the module to recompile. it would be much better if this thing could read something like CmsSettings["PublicSiteOffline"] or something similar to that so that changing this setting would have an instant result.
I'm certain that other global settings could be set this way as well, and centralizing them into the Administration section (as opposed to having to develop a whole module just for this) would let users be able to quickly define global fields with minimal coding.
BTW if anyone is interested the way I handled this was to make a blank file called offline.true. the module checks if this file exists it proceeds to redirect. To bring the site back up, we rename the file to offline.false, so of course, then the original file no longer exists and the redirect is not executed. I imagine this is similar to how your global appOffline.htm works...
anyway just an idea, thanks for listening!