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[quote=Edchris177][quote]And when it comes time to removing the plate (why?) grinding off the head or nuts of each bolt is easier than doing the same to a simple rivet?[/quote]
The techs NEVER grind off rivets, at least not in the aerospace industry. You drill them using a bit slightly smaller than the shank diameter, to depth just equalling the head. Then insert another bit,(using the non cutting end) into the hole & pop the head off. The rivet is then pushed out.
If you drill past the head you will invariably enlarge the original hole, resulting in a weaker fit.

[quote]I've never used an anvil and ball peen hammer on aluminum rivets, so I can't speak to how that works, but I guess it might be easy enough. [/quote]
Don't over think it, "anvil" does not mean one of those 200lb chunks of iron. It can be something as simple as a small block/bar of steel that may be simply held against the rivet end if the work is being done on a vertical or inverted piece of work. You may hear them referred to as a "bucking bar".
In the case of a clew plate, we can always work with the "anvil" on a concrete floor, & the rivet shank vertical. It is then very easy to peen the rivet, or for a neater job, use a medium diameter pin punch,(obviously with a flat or concave end) striking the punch with hammer. Try to do the job with only 6ish blows.

[quote] seems like you would need to clamp the two plates to the sail to keep them in place while you pound each rivet.
[/quote]
Correct, use screw clamps or vice grips,(padding as necessary to prevent marring plates). Remember it is the pressure between the two plates that actually holds the sail. If you don't get that pressure the rivets will rip right through when you put a load on.
Start riveting from the middle & work outwards, or from one end, that allows the plates freedom of movement. It seems obvious, but I've seen people start from both ends, leaving the center till last.
Way back when I flew bush planes the techs did many hours & thousands of rivets to repair damaged skins & floats. They had small spring loaded devices called cleeko's (sp) that were inserted into rivet holes, sometimes hundreds of them, to keep the pieces accurately in place on compound curves. The most junior guy was sent inside & held a bucking bar against the metal, just to the side of the river shank, then called "draw". The riveter on the outside hit the rivet with a couple of blows. This forced the metal pieces tightly together. The bucking bar was then placed on the end of the rivet & the call was "rivet". 
BRRPPP, six blows from a pneumatic gun peened the rivet. You can do the same thing with a hammer, experiment on a few rivets to find the strength of the blow you need to set the rivet in appx six blows.
Go to any shop that maintains aircraft, small ones or jets. They will have bin fulls of high quality rivets & you should be able to pick up a few dozen for next to nothing. Show them the clew plates & the thickness of the sail to ensure the right diameter & length. IIRC the older Nacras used 1/8" (.125) diameter. I got a bucket of various sizes from the Boeing metal shop. 
They do age if left on a shelf for years, you are supposed to bake them in an oven for 20 minutes before use.[/quote]

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