A1 - This is kind of a head-scratcher for a former San Juan builder and North westerner here.  Zinc (anode) is used as a sacrificial material to preserve other more important metals that are on the boat under water.  There is no metal under water except the centerboard and lift cable, which is always wet, and sometimes the lower rudder fittings.

The stock board is galvanized steel.  I'm guessing most of these have lost their zinc galvanizing layer (others can weigh in here), and your board likely looks pretty gnarly after these 50 or so odd years.  I switched my board to stainless steel (Alaskan Copper and Brass Co. in Kent, Washington).  The lift cable is stainless steel with a nico-press swaged eye to connect to the board.  That area should be inspected and refreshed periodically. 

Here in the Pacific Northwest, barnacles and mussels are probably the larger problem.  If I moored the boat, I would raise the center board and detach the rudder to keep it from wearing out the fittings and growing things.  Raising the center board reduces the slop-banging in the trunk, that wears parts, including any galvanized coating. 

A couple of summers ago, we towed a 35' live aboard with a dead engine across Puget Sound.  We wondered why it was so slow.  When we arrived at the dock there was a 6" growth of mussels on the bottom, so I hope you have good bottom paint.

There are a lot of posts on this forum about stuck centerboards, a good place for mussels and barnacles to grow.  What a bunch of horror stories.  (Tech Tip B01).  When you do your periodic haul-outs (yes), you should not to forget to clean and repaint the inside of the centerboard trunk.  Perhaps make it a practice to replace the centerboard cable as well.  Cheap insurance.

If I was going to add a hull zinc, I think I would adhere a small plate with a threaded stud near the stern where it could be mounted and inspected and replaced as needed.  Perhaps it would last several years.  I wouldn't drill a hole in the boat unless someone pointed a gun at my head, and you might learn valuable information.  Your outboard should have a zinc already.  That's my two bits.  Keep sailing!  Glen.

A2 - I too have never heard of a zinc on a fibreglass SJ23.  It is not documented in the manual.  See Tech Tip H06.
However, it might be a good idea to have one in salt water to protect the steel center board and SS lift cable.  Having said that, I don't know how you could install it on the board given the narrow space of the keel slot.  See Tech Tip B01.  Bob Schimmel