“What should I tell my hosts and how can I be part of the mitzvah of hamotzi?”
If there is some sort of bread that you CAN eat, then ask if they mind if you bring it along (just make sure it’s an acceptable hechsher for that family). If you cannot have cannot have any hamotzi, then just let them know. Most hosts are very understanding if other people’s situations and are quite willing to accommodate as much as they can.