Last week, we had an ADR for lunch at Le Chefs, but they were taking walkups. So, if you want to go for lunch, you would probably be ok.
Also, last week, we saw walkups waiting an hour for lunch at Mama Melrose.
As for Boma, if you really don't want to make an ADR, your best bet would be late .. after 8:30 or 9 pm. You can get a beeper and have a drink in the lounge. But, frankly, I would make an ADR.
You may get into Marrakesh with out a ADR, also San Angel, maybe lunch at Alfredo's and Plaza.