I've gotten a chance to work on this project again, and it's going pretty well. Roofing nails are cheap, but the nippers I got were nowhere near sufficient to cut them down. I went ahead and ordered two pounds of 3/16" diameter copper rivets from McMaster-Carr, which is by far the largest expenditure so far (about $20). They fit the holes I drilled in the plates just right, and go through the holes made by my awl with just a little finessing. I started out peening them over, but eventually I decided to just whack them flat with a two pound sledge. The ends flatten out into nice cylinders with much rounder edges than the peened rivets. I'm hoping the fabric survives well, because it'll be darn near impossible to get the rivets out of the plates to re-use them.
The two layers of the fabric shell were sewn together with a series of vertical stitches about 2" apart. I started out trying to measure them, but after a while I was able to eyeball them without much trouble. I bound the edges with the fulled green wool that I still haven't run out of. The eyelets are reinforced with steel rings and bound with white silk thread. Not the most attractive eyelets in the world, but hopefully they'll hold up well. They won't be laced and unlaced very often, so mostly I just don't want them to rip out from general strain. I've got about half of them done so far.
By far the most nerve-wracking part so far was when I finished binding the eyelets on one of the front sections and started putting in the plates. I had test-fitted the fabric shell by itself, and the edges met at the front and the side openings with a little room left, so I'm not too worried about the plates making it too small. Still, I had no guarantee that the plates I'd designed would really work. Fortunately, I got all the plates in with no problems, though I had to trim the side plates down by about 3/4". The copper rivets are sturdy, and setting them is hard on my poor out of shape arms. I experimented on one of the side plates with some copper burr rivets I found in the shop, but they actually ended up ebing harder to set, since I had to trim them to length. There's very little visual difference on the outside from the other rivets, so I don't think anyone will notice. Now it's just more of the same until I get all the plates in. Then I sew up the shoulders, add straps and buckles, and put in some leather-reinforced holes for attaching spaulders.