I ended up leaving well enough alone. I did remove the fibreglass from all of them, except the one in the head and inspected them all. I also cut one out and removed it. They all looked absolutely fine.
I had a very knowledgable and seasoned ocean racing rigger come have a look and he thought they looked like they were in very good condition. He explained that the stainless steel and corrosion is more prevalent in the tropics. He also said that the biggest weakness on a chainplate usually occurs where the chainplate enters the hull. This is where you are most likely to get the standing pool of water that could eventually cause corrosion.
The one chainplate I cut out (which took forever) looked really good. There was no sign of fissure cracks or corrosion. The rust color on the plates came from non-stainless steel screws that held the chainplate box cover in place. There was a little rot on one of the knees. We built up the one knee again and re-glassed them all back in the way they were. I made one small modification though. We left a sliver of the chainplate exposed right at the top so that I could feel for moisture or see signs of leakage. So far, a couple of gnarly sailing trips later with some good winds and all is still fine.
I have some pictures that I'll get around to posting.
P.S. David - your website appears to be terminated. I hope you had a backup copy of everything as they appear to have lost all data in the termination.