Edit the index.php file, change this:
include("ubbthreads.php");
To:
$_REQUEST['ubb'] = "postlist";
$_REQUEST['Board'] = "1";
$_REQUEST['page'] = "1";
include("ubbthreads.php");
Completely untested, but using the request ubb line is one way of
generating a page outside of the UBB, so it should work here.