The oil changes per schedule would be required to assert your warranty rights in an oil-related engine / parts failure, but wouldn't have anything to do with say, getting a wheel bearing warrantied. Also, you never have to get your oil changed at a Mini dealer; you can go where you'd like, or do it yourself. As long as you can produce receipts showing the maintenance was done, you don't void that portion of the warranty.

Having said all that, it is far simplier to have the scheduled oil changes done at a Mini dealer, which should really cut down the arguing about who did what wrong and where in the event Mini has to do some engine-related warranty work.