I certainly can see a company considering someone, with poor social etiquette over a lunch interview, as a person unfit for the work environment. Poor social skills indicate not only a lack of professionalism, but also a lack of respect for other people. It is that lack of respect that leads to workplace strife and all sort of issues that disrupt the work environment.
What kind of lunch etiquette would be considered bad enough to the point where you think it should affect the hiring process? Do you mean like holding a fork in your ahand in a way someone would consider improper or something more disruptive? Where would you say, when it comes to behavior during lunches, that you would draw the line such that crossing it is legit disruptive?