MultiDBLoginService is really just one (and not necessarily the best) way to do this, anymore. First, your service has of course complete control over what connection to use, and you can change the name in the BeforeAcquireConnection (or similarly named) event.
Now, the second questions is, do you want your client application to choose a connection at will, and of so how. Using MultiDBLoginService is one way to do this, but i would recommend using the LoginEx method in the regular login service, where your client can pass any arbitrary number of parameters to the server (one COULD be ConnectionName=foo). Its seldom that assign the name of the connection from the client is really the right approach. Maybe you want to consider making the connection choose dependent on the login (users Foo.* will use connection A, users Bar.* will use connection B). Or maybe you want to choose the right connection based on some more “abstract” flag (for example to choose via Production and Test database, via UseTestDatabase=YES/NO in the connection string).
these are just ideas and food for thought, of course; how to best handle this depends on your exact needs for your project.