Quering by user is in bug 19195.
It was assumed (from IRC conversations) that getting the user id is not easy (requires multiple requests), however from what I can see it's not readable at all. Not even with multiple requests.
list=allusers, list=usercontribs, list=users, none return the id by default or with a property.
Roan wanted to implement this right away but had to leave. Will file here for him later or for myself if I have the time.