Sliced chicken breast and hold?

If I had to do it, if possible I'd slice to order, and hold in chicken broth. If you have to pre slice, I would also hold in chicken broth to keep it moist.

Have you tried boneless skinless thighs? I use them all the time and most people think it is breast meat. As suggested, hold in either broth or melted butter

Get 6 or 8oz breasts, cut them in halves or thirds, shingle them in the pan with a liltle chx stock and hold them. Just because they want sliced, doesnt mean they can tell you how thick to slice them.

I just saw this and I'm sure your event is over but if you get the request in the future this might help. I brine the chicken breasts (yes they are boneless, skinless as well) first for about 45mins. in a simple brine of 1/2c kosher salt, 1/2c sugar and 1g of water. We thin slice the chicken and serve it as a bbq chicken sandwich with bacon, cheddar cheese and lettuce and our customers love it. Haven't had one complaint (knock on wood) that it's been dry.

This is an excellent question. With that Brine, can I do it over night? I am doing an event this weekend for 120 and need to do 30 pounds of boneless skinless chicken breasts. I was going to brine them overnight, then start my cook on site 2 hours before serving time.
Is brining overnight too much?

Depending on how much salt is in your brine, yes it can be too much. A typical brine of 3/4 cup of salt (plus sugar and other stuff) to a gallon of water would be ok for about 4 hours max IMO. Even this may be too much for some people, but if you have time to experiment, go for it.