Till about 2006, the lunch "interviewer" was able to put in feedback, but was specifically told that they were not supposed to do it.
Except for some smart alecks who would put in "Candidate held fork in left hand, was neat eater" I never saw anything beyond "n/a" or similar non-feedback. Most often I would see a savvy recruiter pair up the lunch with the referrer so they could catch up with their friend and it would act as the 'sell' part of the interview slate.
I can say that on my June 2012 interview at the GooglePlex, the engineer showed me the paper on his clipboard and it had a feedback section but said "(not required)" next to it.
He joked that "Well, if you punch a hole in the wall I might write something". Other than that we just talked about hacking game consoles & Chromebooks & breaking out of Chrome's sandbox to reach the OS from javascript.
Most often I would see a savvy recruiter pair up the lunch with the referrer so they could catch up with their friend and it would act as the 'sell' part of the interview slate.
This (lunch with referrer) happened to me when I was interviewing there and I agree that it is a smart move. It put me at ease. Presumably by the time you are there for an on-site, there aren't any questions of fit left.
Except for some smart alecks who would put in "Candidate held fork in left hand, was neat eater" I never saw anything beyond "n/a" or similar non-feedback. Most often I would see a savvy recruiter pair up the lunch with the referrer so they could catch up with their friend and it would act as the 'sell' part of the interview slate.