Upon signup Uber should present options, like "user is accompained by a guide dog"

Then drivers are informed and free to accept or refuse such requests.

Edit: I don't get the hate, when you buy a plane ticket it's the same thing, it has nothing to do with discrimination, it has to deal with being able to accommodate them.

So if the person can't fit a service dog in their car, they won't have them service that call but send another car that is better for the guide dog.