We have just started doing this for DS again. Previous attempts failed b/c even with a reheat it was still mushy from microwave. But we got one of those special thermos products that keeps food quite hot for a while and suddenly it works out! I put some veggies in there too. I cut it up for him so it fits in the thermos/ And yeah, it's kinda weird putting a sandwich in a bottle, but it works!
Got this rec from a DCumer on a post about daycare doing peanut free and thus we had do send something other than PB&J. We are using a Thermos Foogo container but I'm sure there are others out there too.