The key here is to use firm but ripe tomatoes, otherwise they'll turn mushy when you cook them. Don't worry if the skins split, but do be careful when turning the tomatoes, as you want them to remain intact. Crush them into the bread while they're still warm and let the bread soak up the juices.
Note Indirect grilling is cooking away from the heat, using the top rack. It's important to have your coal base well established and glowing before adding woodchips. Refer to your barbecue manual for instructions.