I've shot DNG+Jpeg a few times because I had to quickly edit the images afterwards and it saved computer time rendering the DNGs... I agree I thought Leica's Jpeg colour palette was very natural, pleasing and very much emulated the look of certain films. If that's your end goal, then I think it could work.
I haven't done it because I found the Jpegs to be typical of most in-camera Jpegs, giving up the last x% of image sharpness/detail that could be extracted from DNGs. I also like the 'granularity' of the M's DNG conversions... the fine sprinkling of grain/noise even at base ISO that is inevitably smoothed away in Jpegs, which instead feel a bit waxy. I also found the default Jpeg settings to be quite high contrast, resulting in fairly harsh looking images. I suppose that could be tweaked with the in-camera contrast setting.
Here's something to consider. Shoot DNG+Jpeg. Process the DNG to taste as a B&W image. In Photoshop, bring in the in-camera Jpeg as a layer above the B&W conversion. Change the Jpeg's layer to colour so that the luminosity of the underlying B&W image is 'tinted' by the colour of the Jpeg image. Maybe it will give you what you're looking for, in respect to colour range. The idea is you can get the luminosity tonality you want, the sharpness, highlight/shadow recovery, etc. from the DNG B&W conversion, with the Jpeg supplying the colour for the image. I think for this to work, however, the pixel dimensions of the two images need to be exactly the same and I haven't checked to see if ACR or C1 export at the same pixel dimensions as the in-camera Jpeg. Sometimes the raw converters give you slightly more (or less) image area...
I'm kind of tied up right now with a pile of work and don't feel like taking the time to actually test this myself, so throwing this out there for anyone else to try...
BTW, I doubt it will result in the exact same look as the in-camera Jpeg, but in respect to colour range/character, it might work.