I have done some business lunches/dinners during my MC's and it can be done without going off the Master-Cleanse if you plan ahead both physically and mentally. Physical - be sure to bring enough of the drink. Mental - tell yourself that you're not going to eat and plan to do something else for a few minutes, if possible, during the peak meal time. Sometimes I go to the bathroom, another time I left for an entire half hour to run an errand.
All that said, during my last MC, I unexpectedly had to go to a business lunch on day 9 for which I was neither physically or mentally prepared. I ordered french onion soup and ate just the soup (not the cheese on top.) I still felt sort of sick later and I know I compromised my cleanse, even though I went an additional 3 days.
So the moral of my story is to BE PREPARED and you can accomplish anything.