I'm not sure what type of emergency  you are considering that would take down
your sharepoint  site, but leave your HTML site  accessible. You would
probably be better off hosting the HTML
site on a different server, preferably at a different physical location.

For redirection to another server, you might look at tools such as F5 Big IP
or other network load balancers. Most of these products can detect when a
server goes down and automatically redirect the users. For a simpler and
less expensive capability, you can simply use a DNS alias to point to either
the SharePoint server  or the web server, with a script to toggle DNS between
the two.

For same server redirection, you can tell IIS what URL to point users to. So
if SharePoint goes down but IIS is running, you would be able to go into IIS
Manager and change it. See
www.microsoft.com/.../6
b855a7a-0884-4508-ba95-079f38c77017.mspx?mfr=true for an example.

There are many more ways to do this, some of which are much more
sophisticated. I'm sure the other list members will chime in here with their
favorite methods.