I never found a way (a command that could be sent to cause a restart).
So what I did was to plug the power brick into a timer. The timer is programmed to switch off the power once every 24 hours for about 1 min and then switch on again.
That was my solution and it works ok. There are available some "ip" controlled power switches, which could serve a similar purpose, but they are expensive.
If anybody knows a command that can be sent to a Slingbox to cause it to restart, then tell us!