As of an hour ago - yes.
Previously I was on 22.214.171.124.
if your webmail/ directory is in /var/www/ and your client's root web directory is in /var/www/clients/client1/client1web/
then just type:
ln -s /var/www/webmail/ /var/www/clients/client1/client1web/webmail
Then when your client goes to www.client1web.com/webmail
they'll get the squirrel mail login screen