RewriteEngine On
RewriteBase /
RewriteCond %{DOCUMENT_ROOT}/unavailable/unavailable.php -f
RewriteCond %{REQUEST_URI} !^/unavailable
RewriteRule ^.*$ unavailable/unavailable.php [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]