I am happy to report that I was VERY happy with my dining experience at Fogo de Chao! (And proud of myself for sticking to my Whole30 despite strong thoughts of "it's a special occasion, would a little wine and a piece of chocolate one day really hurt?")
Anyways, I told our waiter about all of my dietary restrictions right away. He was very knowledgable and immediately told me what I could have as is (the Picanha, filet mignon, rib eye, top sirloin, bottom sirloin and beef ribs), what they could modify for me by cooking in only sea salt (the lamb legs, the lamb chop and the chicken) and what I should avoid all together (the parmesan pork loin, the sausage and anything wrapped in bacon). He made me feel completely comfortable asking, and even walked me through the salad bar to point out what I could have and what I couldn't. This was really helpful because I wouldn't have had any idea about a lot of the cooked and marinated veggies (for instance, the beets had added sugar but the zucchini and string beans were just in olive oil salt and pepper). Anything they didn't know (like does the smoked salmon have sugar? it didn't!), they found out the answer right away.
All of the servers seemed to know my requirements, but all were very subtle about it. I didn't ask them to make chicken special for me (who wants chicken when you have lamb and filet mignon?!), but they made TWO batches of lamb legs with just salt and it was awesome! I had more than enough to eat, but didn't feel disgusting like I would after a heavy carb binge. I definitely recommend Fogo for Whole30 dinners out!