It seems odd to me that a Sampson Post or Mooring Bitt could be
attached to the keel in any way, in that the keel is always
considerably aft of the bow, where these mooring 'posts' are located.
More likely that the SP (generally two, parallel wooden posts
separated by (say) 6" - 8" and typically found on a cutter rig with the
bowsprit member between them) would attach to a forward bulkhead, with
the length of the SP terminating at the stem. This will mean cutting
two holes in your deck (for the SP's to pass through, and assumes you
have a substantial bulkhead in an appropriate forward location).

Why not use a Bollard, with a substantial mounting plate *underneath*
your existing deck. In this way, you only need to drill 4 mounting
holes?

For Bollard example, see: http://www.vetus.com/

MW
Los Angeles