Well, the real answer to this is no, there are no techniques that allow safe anchoring
in thin sand over coral or other rock surfaces... especially with a plow. Plow type anchors rely on burial to achieve holding power. In thin sand, burial is not possible, so that answers your question emphatically IMO.
It is a problem in that one can not determine the thickness of the sand layer by observation from the surface. The only method that works is to drop the anchor and attempt to set it, using a lot of power in reverse. If it properly buries, you are anchored. If it does not bury you will drag along... go elsewhere and try again..