One option would be to tunnnel the request through an existing protocol the server is running. You could:

• Invoke a CGI on a web server (just start job if it's long running)
• create a flag file in a (hidden, locked down) directory on a file server, checked every N mins by a scheduled job on the server.

Bit crufty, but should work easily (and both methods provide good auth support, which you'll presumably want).