Leica did produce an R-M adaptor some 30 years ago, code: 22228. It does show up occasionally at eBay for about USD200. It does not couple the R lens to the RF of M camera (hence you need to scale focusing.) In fact, it was originally intended for R lens to be used on Leicina movie cameras.
BTW, Novoflex offers a similar ring for about S$150 - 200. I believe it is still in production.
