It's a bit messy, but if you go into the EPG and schedule a dummy recording you will be offered a reboot.
I did request a restart button (with appropriate warnings of course) on the diagnostics page, but AF hasn't got around to it. I'm guessing, but I think if you were to type "reboot" in the diagnostics command box, that would do it (I'm not going to test it right now - I have a big file transfer going on).
Beware there is a risk the box doesn't always boot up again after this, so best reserved for when manual intervention is available. It also won't be going through its normal shut-down procedures, so if it does get stuck in standby there's a fair chance its wake-up timer hasn't been set up properly and it will miss its next recording event if you don't do something about it.