Krubin - it sounds like your workflow is pretty much like mine. My pictures are in folders on my desktop until they are captioned and ready to store. Whereupon, the entire take, including outtakes, selects, and exported JPEGs is copied to my outtakes external drive. Then, I delete all the "no-stars" and generally tidy up a little, and move the selects and their exported JPEGs to their permanent home.
Up to that point, there's no need of DAM functionality at all. Everything can be done in Photo Mechanic (or my RAW converter/editor) or the operating system. There's really only a brief period in between captioning (when DAM becomes possible) and when the files are moved to their DAM home. In the rare instance when I might need to find something that hasn't yet been committed, and just glancing at the contact sheet won't do, I can use Find in Photo Mechanic. Of course, ranking, sorting, filtering, ordering, and that sort of thing are all ordinary Photo Mechanic functions. Nothing DAM-y there.
All that said, if you have images in that "pre-DAM" limbo state for a long time, I could envision setting up the directory tree where they rest as one PM+ catalog and the permanent DAM area as another. Just drag them from one place to the other when the time comes and a re-scan should take of everything seamlessly.
The re-scanning seems to be the sticking point. Kirk hasn't decided on which approach to take. But worst case (in my worldview) would be we have to press a button to make it happen. Or it could be automatically triggered. It's not going to be much effort either way.
The theme here is that it's something of a paradigm change from most of the all-in-one programs like Lightroom and Capture One that make you commit your whole take to a DAM before you even work on the images. That seems way unnecessarily cumbersome to me, since 90% of the files will never be captioned or keyworded or developed and will just end up in outtakes cold storage, if not the bit bucket.
In a files and folders workflow, like those that have traditionally used Photo Mechanic, pictures are prepared up to the point of DAM-ing in an unstructured world where you just don't have to care about things like maintaining folder-to-folder or folder-to-catalog relationships. In a PM/PM+ workflow, those things won't matter until very near the end of the workflow.
Now, in today's world, many of us do use non-destructive RAW converters instead of Photoshop (so Photo Mechanic > RAW converter > Photo Mechanic Plus) and in some cases (Lightroom and Capture One, but not ON1 RAW) we do have to worry about maintaining those relationships in those programs. That means a Capture One or Lightroom user doesn't want to break the relationship between an image and its session or catalog, lest all the edits be lost. Thus, the move from staging area to final home might need to be done in the RAW converter itself.
PM+ is agnostic to all that. It doesn't need to know or care. Once the images arrive where they will live, it can scan them and be good to go.