Although this is a fairly old topic, we had a feature request for this in trac.
While doing a LinuxMCE install on Cyprus the user requested a similar feature. And after thinking a bit about it, we found out, that this can be achieved with a dedicated egg timer device. The egg timer receives the device in question to be turned on, as well as the amount of time in seconds before it should be turned off. What is different from the use of Delay is, that if the event that started the egg timer (or different event that would extend the egg timer) happens, the egg timer gets extended and does not turn off after the first period of time.
If anyone is interested in testing this new device, please download http://deb.linuxmce.org/possy/eggtimer
into /usr/pluto/bin, and add the Egg Timer device below your CORE.
Feedback is greatly appreciated.