Can you do a "1-of-1 or 2-of-2" transaction, where either the overseer spends the coins, or both the overseer & the holder spends the coins?
You can do 2-of-3 (overseer holds 2, so can always spend, you hold 1) instead to accomplish the same thing. I suppose having the overseer put transactions in the block-chain that everybody can see might make you and the people you're paying trust them more...