Good question, Charlie.
Any bass player I know would see that progression, recognize a descending bass line, and play the D. Would a computer algorithm? I don’t know, so I would like to make it perfectly clear.
Would you write D/C..bd on a leadsheet? No. Would BIAB understand and use that expression correctly? Yes.
By the way, on a leadsheet, it’s fine to write only the slash root if the underlying chord remains the same. So you might see Dm and then later in the measure see just /C and you know this means keep playing Dm but add the descending bass note of C Can you do that in BIAB? I don’t think so (I need to try it).
Thanks for your reply Matt. It gives me a better understanding in regard to notation. Presently, if I'm in need of a D/D or the same with any key, it's a simple copy/paste for me to do in just a few seconds using the BIAB Audio Editor to edit a root D bass note to the correct position but an avoidable edit if PG Music were to do as suggested and make the D/D command possible.