Therapy registration is not free and it is not automatic. National organisations run temperament evaluations, require a handler-and-dog team to test together, charge registration and renewal fees, and generally provide liability coverage for approved visits as part of that. Expect an evaluation fee, an annual registration, and a health certification from your veterinarian confirming the dog is fit and current on preventive care.
The assessment is where a giant breed needs preparation rather than good intentions. A dog rated 3 out of 5 for trainability is responsive to consistency but not automatic, and the specific skills tested — remaining settled, tolerating handling from strangers, ignoring dropped equipment, walking politely past a wheelchair — need to be taught deliberately. At this weight, polite is a physical requirement rather than a courtesy.
What makes it a good fit is the energy profile. A visit is an hour of being calm in a strange building, which is exactly what a breed documented at 2 out of 5 for both energy and exercise requirement does best. High-drive breeds often find that hour harder than a long walk.