You seems to have good lenses, so if you look at old film lenses should be something really unique.
I have Canon 40 2.8 pancake and even with adapter is small enough for M.
You can also wait for native 50mm lens for M.
It depends on what you mean by "worth it". As you may have noticed, lots of vintage manual lenses are cheap, and lots of those can be very good to superb; it's an inexpensive way to play around with a variety of lenses. Given the tiny size of the M you'll likely want small lenses. If so, Pentax is perhaps the best place to start - a few weeks ago a bought a Pentax Super-Takumar 50mm 1.4 for c. $100 which makes excellent images, and an even smaller Pentax-M 50mm 1.7 for less than half that. Check out the pentax forum for vast numbers of user reviews. Your 100L is a marvelous lens, of course, but you might rather use something a lot smaller, such as the superb Nikon 55mm 2.8 macro (if you want to get close) or the Nikon E series 100mm 2.8, which looks tiny next to a 100L. I recently bought both those lenses in excellent shape for c. $120 each (and could have paid less if I had tried harder). With the right adapters, the range to choose from is vast. (I use them on my Olympus M43 and Sony a7r & a6000, but I don't see why they wouldn't work at least as well on an EOS-M.)
Thanks for your suggestions. You're right, I would like to try a light/small lens on my M.
I also have to consider that a new 50mm Canon (F/1.8 IS?) could (will eventually) be delivered, so I'm not sure about having (in the future) two prime lenses with the same focal lenght.
Maybe something different (the 100mms you mentioned, or a good 35mm...).
I recently installed Magic Lantern on my M and appreciated the improved ability of manual focussing, thanks to the focus peaking feature.
What is your impression on the Magic Lantern?? Did you see any problem?? Which built day did you used? I am thinking about installing magic Lantern on my EOS-M. But I have also read that there are problems with the shutter release. Thanks for the advice in advance.
No problems so far, but I just played with it two hours, so I'm not (yet) a reliable tester.