I participate in a dog training forum, and a couple people there have trained their dog to use a litter box. In both cases, the owners are older women, and the dogs are very small.
Personally, I don't like the idea unless the litter box is outside on a balcony or enclosed patio or something like that. If inside, you are telling your dog it's ok to go potty indoors. I don't want to teach my dogs that.
But I am home all day so they don't have a problem with having to wait all day; they are taken outside in the early morning, late morning, mid-afternoon, and after dinner, at minimum.
Probably, if it's any consolation to you, Marty is likely to get used to his new routine with you pretty soon. Also, since he was treated for dehydration at the vet's, he might have had a lot more fluid in his system than he ordinarily would, thus the 3:00 a.m. walkies. Once his system normalizes, he probably won't need that any more.
How is he doing? Drinking, peeing and pooping like nice little poodles should?? I hope so!