This would be very helpful! I even suggested a long time ago to have customized sound clips with a star code or what could be called voice prompt recordings that are assigned an extension number. Then the message could be updated anywhere it is programmed in the dial plans.
I always thought it would be helpful for there to a node option in the dial plan that points to the holiday schedule and inside this holiday schedule you can assign the sound clip for the time frame you want. This would also save time as you wouldn't have to log in every time for each holiday to make the change and you don't have to worry about the thanksgiving message playing during Christmas time because you didn't manually change the greeting. It would just happen automatically.
Anyway, great suggestion and I hope this is something they can implement in the future.