Is there someone co-ordinating the trip? Lunches provided etc? They normally ask if there are any food allergies etc if they are putting food on for people. You could email/call first thing on Monday and let them know you have just found out you are intolerant to wheat etc (hence last minute notice) and unable to have sandwiches but are happy to bring along some food if it can be stored in a fridge if it is too late for them to arrange something (have selection of meats) or just inform them of what you can have.
I had to do this once at a meeting (on a previous no carb diet) and was given a chicken salad instead of the sandwiches on offer. If that happened, you could just eat the chicken/protein and leave the salad bits.
You will prob feel a bit hungry, so I'd definitely have things you can snack on during breaks as I am sure there will be other things like pastries for other people if they are anything like the meetings I have been to.
If you are out with everyone at dinner, have a quiet word with the chef/waiters beforehand and let them know the same, you can let them know what you can/cant have in case you don't want to say anything in front of clients/colleagues. So it doesn't look 'odd' to other people, you could have it served with veg, just leave them. People do tend to look a a plate containing just protein and wonder what is going on
There are definitely ways to keep going, you'll just need a steely resolve and keep remembering your goal so you don't fall off the wagon.
Oh and if there are drinks, the old trusty "i'm on antibiotics" can be used
Hope all goes well!
