RewriteEngine On
RewriteCond %{REQUEST_FILENAME}	!-f
RewriteCond %{REQUEST_FILENAME}	!_shared
# Rewrite /sites/[sitename]/[customcode]/file.ext --> /sites/_shared/file.ext
RewriteRule [^/]+/[^/]+/(.+)$ /sites/_shared/$1 	[QSA,L]

# Is default and not found? 410: Gone
RewriteCond %{REQUEST_FILENAME}	!-f
RewriteCond %{REQUEST_FILENAME}	_shared
RewriteRule .*	-	[G,L]