Have the mailing script sitting on the server, in the web document root. In the script, put security (ie limit remote addresses that can access the script, ask for user/password/ssl, whatever).
Now, all you need is one machine to access that server to trigger the sending of messages. That can be a PC anywhere, even your home PC with a windows scheduled task. That way, the cron can sit anywhere, but the work is still done on your web host.