Hi, it does work well on a M body. I have the TRI-E 28-35-50 V2 with the 49mm filter thread and 6 Bit coded by Leica.
I have attached pictures showing the mounting ring with a spring on part of the M mount. It moves in 3 different positions when you go between focal lengths.
On the back screen, you can see it change focal lengths as you switch. It takes about 2 seconds to show up when switching. I guess between the spring doing its thing and camera detecting a new length, it has a short delay to show up. When editing it shows the proper focal length, and in the view finder the frames auto change.
On the SL with M adapter, it gives you a choice when you first turn the camera on which of the 3 to choose from, and then if you change you have to do it via the menu. Which you might want to do so the IBIS knows how to behave.
I took it as my main daylight lens on a recent trip to Cyprus, and the Peloponnese region. 3 from below are from the TRI-E of that trip.
Dave