Sounds like a good idea to me. The fact that it screws to the body will make the job easier. I have thought about doing the same thing, as I have auto dim mirrors on my other car.
With the height of the mini, that would relieve some of the glare from cars with jacked up suspensions, or trucks.
As for the warranty, in the US, the only warranty that would be affected would be the warranty on the bracket, the headliner, and the mirror itself. If you have laws that are contrary to that, you have my sympathy.
Let us know how it works, if you decide to do it. I am going to do it as soon as I find a suitable mirror, and it will cure the "oval mirror" syndrome.
I bought it, I paid for it, and it is my car. And I have no awe of service writers, used to be one myself.
