Hi,
The app_offline.htm is a feature of ASP.NET as Asad mentioned, but I think it is useful to discuss this in terms of 3 scenarios.
The first scenario is taking the entire app offline and this is supported by the ASP.NET app_offline feature. This is useful when you want to restore a db backup or something of that nature, but it doesn't prove useful during upgrades of mojoPortal because it also disables the setup page since it takes the entire app offline.
So the second scenario would be to have a way to take the site content (all sites) basically offline but keep the setup page enabled. We don't currently have a solution for this, but I think it would be a good idea to have such.
The third scenario is when hosting multiple sites in a single installation, when we provision a new site, it would be nice to have a mechanism whereby the site content can be disabled unless signed in as an admin user. So we would need a way to keep the sign in functionality available even when the site is disabled for non admins. This is another scenario we don't currently have a solution for, but it would be nice to have.
I've made notes about the 2 scenarios we don't currently have support for and will try to come up with a solution for them when I have time.
Best,
Joe