Well, 3 years on and there is still no official way to use these cameras from DxO.
However, you can always use other software to first convert to TIFF.
For example, if you have the open-source darktable installed, you can use its command line tool to perform the conversion:
darktablecli --verbose q2m-image.dng q2m-image.tiff --core --conf plugins/imageio/format/tiff/bpp=16 plugins/imageio/format/tiff/compress=2 plugins/imageio/format/png/compress=2 plugins/Imageio/format/webp/comp_type=1 plugins/imageio/format/tiff/compresslevel=6
This will generate the TIFF (and in this case, also correct for the Q2M’s severe lens distortion).
Once in PhotoLab, I believe that you can perform all edits except for Prime noise reduction or any of the lens corrections. If you need these, set up darktable (or equivalent) so that it applies these the corrections you want by default.
It is possible to automate this, so that you can just run a batch script to convert all images in a directory. On a Mac, it is also possible to use ‘automator’ to automatically perform conversion when files are added to a folder.
The main downside is the extra conversion time and the substantial disk space increase implied by the TIFF files, even with compression enabled.
For Leica monochrom cameras specifically, this is AFAIK the only way to use PhotoLab. Tricks such as changing the EXIF or using Adobe DNG converter will not held because the underlying raw data is inherently grey-scale, and DxO will not allow that to be loaded.
I still do not understand why DxO do not provide a generic conversion such as that provided by darktable-cli. While the image quality will obviously not be as good as implementing their full raw conversion, it would at least make the software usable.
My experience with the Q2M, is that the resulting image quality with darktable-cli plus PhotoLab is no better or worse than using, say, Capture One, which does support the camera natively. But with Capture One, conversions “just work” for all of the cameras I use, without the need to find clunky work-rounds.