FYI... In this brief article, the owner of Moor Seacure (most of the balls in the BVIs) discusses reserving mooring ball by leaving a dinghy on it. Reserving a Mooring Ball

".... It is Mr. Moir's feeling that once a mooring ball has a dinghy tied on, that mooring is effectively "in use" ... regardless of the size of the vessel. The presence of a dinghy is a clear indicator that the dinghy owner or lessor intends to return. By design, the moorings system is offered on a first come, first served basis. A dinghy is a boat and subject to the same fees as any other boat, regardless of its size. ....."

I have never done this. Personally, I think it could potentially lead to a stressful situation if other boaters have different views.