I wonder if anyone has any knowledge or experience regarding service dogs? A family member who we hope will come and visit us in Portugal has a service dog that alerts prior to the person having a seizure (and is also trained to lie down across the person in the case of a seizure, to prevent the prone person from seizing down a flight of stairs, for one example), so a serious condition, but not an obvious visibly-identifiable disability like a blind person with a guide dog.
I'm not sure, but muzzling might also be an issue for this type of service animal since it might prevent the alerting and recovery behaviors.
I see in the link Thomas provided (thanks for that!) that there is mention of "assistance dogs" and "accredited assistance dog training establishment" -- if anyone knows more about these details and how they might apply to tourists in PT with such an animal, or has a link to more details, that would be appreciated!