Typically changing the behavior of an extension is ill-advised (as the purpose for doing it, is unnecessary). From a disaster recovery scenario you are adding a step that really isn't needed (the need to change the behavior of an extension so it performs differently).
On an Apache environment, sure you can use an .htaccess file to do it, so it is deployable with your code (not certain you can do that with IIS).
I guess my point is, changing the default behavior of an extension is an unnecessary step since there are extensions that already support what you want to do.
Now don't take my statement to mean you shouldn't perform
// accessed by calling index.php
and your mytemplate.html may very well include php tags.
My statement is directly against using index.html as your serving page and index.html contains php tags and must go through the php parser first (not a default behavior).