May one communicate via sign language (like ASL) with others (whether or not one is deaf-mute) after washing one's hands before Hamotzei, or would this constitute a Hefsek?
There is a popular misconception that the problem with speaking between Netilas Yadayim and Hamotzi is a hefsek. That is not actually true. The problem is hesech hada'as, or being distracted from guarding your hands. [See Shulchan Aruch Orach Chaim 166 and Mishna Berurah there.] So talking with sign language should be the same problem. (Although maybe you could argue you are using your hands so your mind isn't distracted from them.)