After giving it more thought I came up with the following integration idea, which extends the concept of sidecar files and is portable regardless asset type:

• digiKam displays preview from a sidecar JPG file, which follows sidecar naming convention i.e. asset.ext.jpg. That would provide support for versions as well. In case of raw images that would mean a pair of a XMP and JPG sidecar files for each version.
  • The sidecar preview is handled by digiKam equally to sidecar XMP files (move, rename etc.).
  • If there's no sidecar preview, then fallback to display preview from the image itself as it does now.
  • On export operation, the source for export is the sidecar preview, not the original image. Actually, that's what you expect to have exported anyway.
• The sidecar preview is exported from darktable using any preset the user likes, given the location and naming convention are preserved. The JPG export quality settings result in preview quality and export operations in digiKam.
  • Possibly, generation of sidecar preview files could be automated with scripting in darktable. It's my guess, as I didn't take a look on it yet. Anyway, even doing it manually would not be much of a trouble.

I'm going to make a feature request to digiKam developers. I hope it will have serious chance to be accepted as it extends the current concept of sidecar files evolutionary, not revolutionary.