I often bolus a few units when I order (like not many, equivalent to 10-20g depending on what I am eating), and then make up the rest when the food arrives. I always have a granola bar and glucose tablets on me, if the food should take a long time.
As for low carb, a lot of places will offer a "lettuce wrap" for their hamburgers, where they use lettuce instead of buns. It's kind of messy to eat, but it's virtually zero carbs and I actually like it better than a hamburger. I find if you tell a server that you are diabetic and looking for low-carb options, sometimes they even offer to prepare something not on their menu that will meet your needs.
Most chain restaurants have their menus and/or nutritional information online, which is vital for knowing what you are eating and bolusing accurately for it. I always try and have this information on my iPhone/iPad as the restaurants themselves don't always have it available.
I generally avoid Subway, but years ago they used to offer sandwiches on buns sort of like hamburger buns. It's still about 40g carbs but much less than their usual subs which are 60-100+ carbs and (for me, at least) still spike me hugely even when I look up the nutritional information.