Possibly. But since tags have a fairly standardized way of appearing in metadata (xmp), it is precisely the feature that could be implemented in digital asset manager software.
Darktable devs have a lot on their plates, and understandably the focus is on raw processing, since this is where Darktable stands out. Contributing code to the raw processor pipeline usually requires a deep understanding of color science and/or math.
In comparison, contributing to a digital asset manager requires a very different skillset, something that a lot more people have. If the latter was factored out from Darktable, its development could go faster and users of all raw processors could share the benefits.
Previewing edited raw files is a valid concern and is an open technical question. As a stopgap, the Thumbnail Managing Standard allows large (1024px) thumbnails, so raw editor applications are free to save a large preview for culling. Then later on raw editors could agree on a simple api to deliver larger previews from a cache, or generate them on demand.