I just totally disabled my chime on my 2006 R52 by using the Moss Motors "Instructions for MINI Cooper gauge face kit" to figure out how to access the back of my speedometer. The speaker for the chime is on the top center of the back of the speedometer. If you Google the instructions it will show you how to gain access to the speedometer and you will be able to unscrew it from the dash.

Once you do this you can lay it down and see the speaker holes on the back. You do not need to unplug the speedometer from the car. I used a utility knife to cut away the speaker holes/slats to get to the speaker. At this point you can stuff cloth or some other material in the hole to quiet down the chime and cover with duct tape.

Some people might put silicone over the chime speaker slats and not cut into it to help mute it. That way it could later be peeled off and be a reversible mod. I however took a pick and hook set from Harbor Freight and jammed it into the speaker which 'disabled' it. It only took me about 15 minutes to do this mod. No more annoying chimes at all for me whatsoever

 

My chime goes off with my e-cig and iPhone 7 both weighting in less than a LB. I see the gizmo that acts as if there is a seatbelt attached to prevent the warning which is cool but I have passengers daily too so that would be a pain. How about a seat belt extender? This are cheap? Wouldn't that work?

 

Carly (BMWhat app) can disable it also.

 

I've used the Craven seatbelt buckle substitute for a few years with ease and convenience. It's thin and tucks away when not needed. I suppose you could get a buckle end from a wrecked MINI that might serve the same purpose but it would be bulky. The Craven gizmo is slightly thicker than a credit card.