What size hole are you looking to drill?
If you are using a full size drill bit and not a spade bit, I would look at getting a piece of pipe with the same size ID as the bit and weld that to a plate that you could clamp to the post. I think the only issue would be getting a long enough drill bit. Would probably have to order it from grainger or a similar supplier.
Gone, out doing soft recreational past time with overpowered boat!