We have a Joomla installation sitting in the same exact root folder as the client's old system, which is html-based. As it happens, the server will jump on "index.php" before it will look at "index.html" so it works out well as far as ease of deployment. The problem is that we've had to take it down to make some tweaks, and the way I did that was to just rename the file "_index.php". That way, we can still look at it, by putting a "_" in front of the filename and going directly at it, but we have to do that every time we hit a menu item, which is annoying.
Is there a way to tell Joomla that the main filename is "_index.php" (or something else, for that matter) instead of "index.php"? This way we can test all we want, without having to add the "_" by hand every time we render a page.