I knowingly pushed it last night, but I'm happy to report that there is no real damage to speak of--just altered cosmetics.
I had four lamb (loin) chops to sear, and instead of using an 8" iron skillet, which would have been a more appropriate size, I used a 10.5" copper saute. I put a pat of butter and a bit of olive oil over medium high heat and waited for the first wisps of smoke to rise, which indicates to me a good searing temp that is still below the melting point of tin. I laid the four chops in the pan (they probably only occupied about 25% of the bottom of the pan) and left the flame at about 80% full bast. Normally, I'd back it down to medium to maintain the searing heat, but I wanted to see how the pan would handle it. I gave the first side about 3 minutes and the second side the same. As I lifted the chops out of the pan with tongs, I noticed the that tin was shining (like new, non-darkened tin) and smeared where my plastic tongs scraped the bottom. The end result was that I have 8 shining smears on the bottom of the pan were my tongs touched. It looks a little like a thick hand-wiped tin coat, and the rest of the pan retained its smooth tin appearance. No copper is showing through, and other than the altered cosmetics, I can't see any further issue.
So, now I know--find the smoke point of my oil/butter mix and back off. At least there was no real damage in the process. And the chops were delicious--nice seared crust on the outside and beautiful red in the middle.