I had it done.
At the time, I had a sailboat with 100' of chain and a CQR anchor that also needed a spruce-up. A friend was getting his chain and anchor re-done and to get to the minimum weight he needed to convince a few more friends to pile ours onto his. That accomplished, here is what I learned:
1 the chain had to be cleaned in a process that at the distance of 25 years, I recall as similar to being shot with a million shotgun blasts.
2 the process involved inspection after the cleaning and a redo if not clean enough.
3 The cleaning was the most expensive part of the process, but maybe the most important.
4 the re-galvanized chain looked great for the rest of the time that I owned that boat.
5 the CQR came back to me lighter than when I sent it in, as the process removed (melted) the lead weight that is part of the tip of the plow.
6 at the time, the cost was less than the cost of new chain and the look and durability were good.
7 when I got my present boat, the chain was leaving a lot of rust on deck, so I looked into doing the same thing over again. By this time the only shop in the lower mainland of BC had been shut down with the redevelopment of the south shore of False Creek and I couldn't find a shop that would do it.
8 new chain was less expensive than shipping my chain a long way to a shop that could re-galvanize, and I got to lengthen my rode.
9 20 years later, my "new" chain still looks good and my anchor still looks like it could use new galvanizing, but isn't significantly worse. (Tom, I may be interested in the spray you mention).