Is there a bull mastiff rescue group? You could get another full grown dog. From our own experience, I would never get another dog unless someone was at home full time. It's just not a fun life for a dog to wait all the time for those precious few moments that you are there. And for a puppy, that would be awful.
However, I do like your idea of taking the dog to work. if you can get away with it; by all means!!!