This is a very clever implementation of your previous technique. However, if the hypothetical griefers were able to destroy blocks around your chest, it would be very simple to just dig down on top of it and steal your items. If some kind of protection system was used, there would be no need to secure the chest. However, it might be very nice on an adventure map.
I made this trap today. What do you think of it.
1. If you see ~(The OP's name) on a server DO NOT till any dirt!
It lets you hide your chests and reveal them using Tilled soil:
Otherwise, cool trap + hiding spot.