I'm confused. You want your clients to not go out to LiveUpdate, correct?
If true, than you can leave this greyed out as it already is.
Your clients will than connect to and get updates from the SEPM based on the current heartbeat. It is currently not possible to schedule content updates from the SEPM.
The SEPM downloads the updates, when the clients check in according to its heartbeat, they begin the process of updating. No way to schedule this, is just happens automatically.