Interesting, Donald. A web search for Eyton's Tree Duck confirms his I.D. (of course, he should know, huh?). That's a species I'd never heard of before.
I also haven't heard of Brewer's Duck, but the concept of naming hybrids is long established. There are some hybrid North American warblers that occur often enough to have acquired names; often, it wasn't clear whether the hybrid was a new species or not when they were originally described.
I would think that keeping Harlequins would be difficult. They breed on fast-flowing mountain streams and spend the rest of the year along rocky sea coasts. How would someone approximate those habitats in captivity? But perhaps they're more adaptable than I know.
That's quite a resource you have there. Here's a question: do you know if he pinions his ducks? That is, have the wings been clipped so that they are permanently unable to fly? One of the really tough subjects in birding is how to determine if the occurrence of unusual waterfowl is the result of natural vagrancy (I mean that in the biological sense of "wandering", not in the human sense of "homeless") or an escape from captivity. It's often impossible to know, especially in those cases where the bird shows no signs of having been captive (unusual feather wear, leg bands, etc.).
