Quite right too Luke - hard evidence is what is needed!
The Knight and the Blast Furnace: A History of the Metallurgy of Armour by Alan Williams has some interesting passages about copper rivets and armour found in the Sutton Hoo ship burial but it has to be said, the subject has a limited appeal so the book might not be in your public library!
That is the trouble with being an historian, it is very hard to prove anything conclusively unless the artefact actually survives however we do have some 'evidence' in ancient stories.
A good example would be the King Conchobar (Conor) who ruled his kingdom from a hill fortress called Emain Macha near Armagh in Ireland. Legend says that there were three great halls there; one for the kings, one for the severed heads and spoils of war and another for the javelins, shields and swords. Conors hall had 150 inner rooms and 'the walls were made of red yew with copper rivets'.
I did find this picture of a very early comb that was discovered when archaeologists excavated an ancient sewage pit - nice job eh?
The comb is made from antler or bone or possibly a combination of both. Approximately 70 mm long – from centre of end plate and 37 mm wide at the end plate narrowing to 23 mm across centre of comb.
Rivets – four copper alloy rivets arranged in an irregular line but roughly evenly spaced along the side plate.
Teeth – one side are coarse and widely spaced whilst the other are fine and relatively closely spaced. The ends of the teeth are cut to form a sweeping concave curve on each side.
Double-sided combs were made from the about the third to the thirteenth centuries. According to the archaeologists who found it, this comb appears to fall into the last few centuries of that range.
Aparently antler was the preferred material for comb making and iron for rivets and few examples have copper alloy rivets or were made from bone. But, I guess what I am saying is that there is no reason to suppose that copper rivets would only have been used for 'hard' material fastenings. Leather is an obvious material to be riveted. I'll be in touch shortly!