Well, here's the trial on the underside of the front headrest.
So, the light colour wasn't a big issue: it took about 60% power at 60% speed to burn this. When I looked in detail
it was clear that the leather could actually take a lot more detail i.e. I've probably over-simplified the logo graphics. The shiny edges don't show up in normal light: they're a side-effect of the bright LED light I'm using for the photo'.
The conclusions? Well, it would probably look much better on black upholstery - I'm not sure that black on oyster works that well. If I was going to do it on the visible headrest, I'd add a lot more of the original detail that's on the graphic (a lot of the edges are chamfered), as this is what the laser does well. And I'd perhaps make it smaller. I'd probably dial-down the power, but make more passes, so that the edges were even clearer.
I suspect that as well as black leather, matt black trim would be a good substrate for laser engraving: the results would be fairly subtle. The challenge would be any curvature beyond the 5mm (3/8") focus range of the laser.
Re the black lines - my understanding is there are techniques that let you apply colour using a laser (e.g. using a coloured tape which then melts into the substrate - in this case perhaps light brown to blend better with the oyster) but I can't be bothered to do the experimentation and anyway, I'd run out of hidden surfaces to test!
And finally, I'm not sure that a pattern wouldn't look better, on the front of the headrest, perhaps the BMW logo, or something with a local theme (we have a local flower that historically was used to represent the county that might look more attractive), etc. You can also argue that there are already too many ///M logos splattered randomly across the car as it is ....
Overall, though, laser engraving looks like a viable option for decorating your upholstery and has potential for trim too. Now I know the process, I reckon I could get a graphic from PC onto leather in about an hour.