I believe you can only schedule regular task per host right now. You can schedule a “run now” task to wipe the disk, and schedule a 1 time scheduled task to re-image the machine in a few hours. That gets you there, but not very elegantly.
If you are saavy, can you try making an addition to the Fog scripts in the init image to handle a disk wipe and re-image task and then modify the Fog menu to add that option, passing in all the parameters needed. Just keep a backup copy of the original files and your modified versions for when you want to upgrade Fog versions.