RewriteEngine on

RewriteCond %{ENV:URI} ^$
RewriteRule ^(.*)$ - [ENV=URI:$1]

RewriteCond %{ENV:BASE} ^$
RewriteCond %{ENV:URI}::%{REQUEST_URI} ^(.*)::(.*?)\1$
RewriteRule ^ - [ENV=BASE:%2]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)$ %{ENV:BASE}maintenance.php [R=303,L,QSA]