Yes, crevice corrosion. SS even rusts when it gets wet all sealed in FRP. I know this because our aft chain plate pulled up about 1/4" - 1/2" while we were sailing.
When we dug out the knee wall the SS bolts had rusted badly and the wood core had turned to soil. At the time I fixed that one I just re-did what had originally been done. Now I would do it differently and mount the chain plate outside the knee wall of FRP.
You can think about relocating the chain plates to outside the hull for the shrouds and even after chain plate. The main consideration for the shroud chain plates is if you will lose more sheeting angle for the jib
such that it will not head into the wind as well. For now I'm going to keep my chain plates inboard, as they were designed to be by Sparkman & Stephens. I don't feel qualified to 2nd guess these designers.
I like the look of the outboard chain plates on boats that have them. I'd hate to go through with the work (and expense) of locating the chain plates outside, on the hull only to find out that there were unintended consequences. You very well might need longer shrouds to accommodate the new chain plate position.
I just love what the Sabre salesperson told you about the chain plates: "the chainplates were embedded so that any water seeping in from above won't bleed through and ruin the interior woodwork
." Right, the water that does seep in will only slowly attack the integrity of the chain plate! I'd rather do a little cosmetic cabin refinishing work and know that my chain plates were up for the job they are expected to do. The boat builders do it this way because it is easier and cheaper for them and by the time any problems do start to arise the boats are long past warranty and it is your problem, not theirs.
Otherwise Sabres enjoy a quite good reputation.