A friend and I were doing some repairs to a Cessna 140 about eight years ago and did some solution heat treating with an oven to some #6 AD rivets to make them easier to drive (some were back-riveted in tight areas). We did pretty much what you guys described; heating up to around 900F (I forget the exact temp) and quenching them in water. The rivets were definitely easier to drive after that!
I was curious about the effects on strength, and built a "pull tester" out of two four-foot lengths of angle iron and a fish scale, along with several identical aluminum plates to rivet together and shear apart with the levers. It was pretty similar to what Bob described (great videos BTW). Here's what I found:
1. Right after quenching, rivets that had been heat-treated had around 20% (IIRC) less shear strength than rivets that had not been heat-treated. That was expected anyway because the whole reason for this was to make them softer (hopefully temporarily).
2. After several days, the heat-treated rivets regained practically all of their shear strength if not driven.
3. After the same number of days, heat-treated rivets that had been driven were about 10-20% weaker than untreated rivets that had also been driven.
So it seemed like, after several days, the treated rivets were about as strong as untreated rivets, but only if they had not been driven. If the heat-treated rivets were driven, it seemed as if they never regained their full strength (at least not after about a month of waiting). This happened reliably with several samples. It's as if the work-hardening from driving somehow interfered with the age-hardening process.
It would be interesting for someone else to try this and see if they get the same result. For me, I was convinced enough that I spent many hours drilling out the heat-treated rivets and replacing them with untreated ones.
Steve