Hi, im a volunteer for the cinnamon trust who foster animals, including dogs, when elderly or terminally ill owners are in hospital. We've fostered a few dogs for them that we had in our home for upto 2 months each. Also they are looking for people to walk dogs for elderly people who can't themselves. Its not exactly what you describe you're looking for but thought it still might be of interest. Their website is The Cinnamon Trust - The National Charity for the elderly and their pets