Yes, the way I envision this working, is that KeyNotes would have a similar ability as RefNotes, to be marked as a Notation, or a Length, Area, Volume, or Object. For a Length, Area, or Volume, it would allow selecting a Layer, and an Object would allow selecting an existing Block. In this fashion, it would make it easier to convert a legacy detail.
One feature I was mulling, was if you might want the quantities multiplied times the number of Detail Callouts, or some other fashion in which you could indicate how many of those detailed areas exist.
The challenging part is the fact that the detail database and the keynote database are wildly disparate. On a single installation, you wouldn't notice it -- but once you are working with multiple Preference Sets, or transferring details between offices, suddenly there becomes the problem of a detail not having the keynote data that is called out within it. So in short, it seems it pushes towards a long-term engineering change, to have the keynote data replicated within the detail database, to give the details more portability, and provide a great deal of data integrity to the keynote data. In the meantime, however, it shouldn't be too difficult to remember that all the keynote data is stored within the Preference Set, and should be backed up and/or transferred along with the details.