Fixed formatting (please use preview).
As is, I can't tell where we'd use your block of code, what are its advantages and drawbacks, and whether it is backwards-compatible. We're web developers, not DBAs :) All I can say is that it is far from trivial.
As a consequence, this ticket is likely to fall into the "yes, Django could have been designed differently, but it isn't" bucket, but I'll leave it open for now in case someone's interested.
If you want to see this in Django, you will have to 1) produce a patch 2) explain why it is useful — your explanation is mostly hand-waving at this point.