Duckworks sells a short piece of chain with a crosspiece at the lower end made for this purpose.  One simply tilts the crosspiece along the chain and lowers it through the socket. 

   I glued a small triangular block directly under the oarlocks. It has a 1/8" hole drilled thru it. A 1/8" line is looped thru that oarlocks pin hole you mentioned and then passed thru the block. This keeps the oarlocks tied down and not going overboard. I didn't go for the chain because I thought it would mar the finishes. The soft line is working good for me. Sorry I can't post pictures here, but if you want to email me I can send you pictures that way. [email protected]

I went to Duckworks site and was not able to located these chains. Any chance you could post a link?   

   Found an old post. Use keyword oarlocks. Look for post oarlocks chains vs wooden toggles. Dated 6/15

   A number of marine suppliers carry the safety chains.  Hamilton Marine is another online with them:

https://shop.hamiltonmarine.com/products/oarlock-safety-chains-brass-pair-33964.html

I just used some small line tied to the ends of the locks, and whittled a couple of small wood toggles from some scrap bits from the rails for the ends.  They just have to be small enough to go thru the socket the long way.

  The chains at Hamilton are the same as what I was referring to but significantly more expensive (worth it not to lose an oarlock though). I couldn't find the chains on duckworks but it is where I got them; however a few years ago. Perhaps they made a change with the business move.  I'd give them a call.

I simply use a stainless steel keyring through the hole on my bronze rowlocks. The ring won't go through the hole in the brackets. It's a simple fix. My boat is a Skerry though so perhaps a NE Dory needs a more complicated solution.  ;)

Bronze rowlocks are too expensive to lose over the side.  

If cartopping the dory an easy way to remove and secure the oarlocks would be desired. Maybe the chains would be too dangly? Maybe just a simple twisty wire would be a cheap and cheerful solution.

   One of the benefits of the chains is that I can pull up the oarlock and let it hang inside the boat so things don't snag on the ones not in use as well as for putting a cover over the boat.  Also makes it easy to remove them for tranporting; I've found they move around too much when i am trailiering.