Hmm, as I said in the other thread, you may find that this is how it works already...
Unless I'm completely wrong, all animation IS relative to parent space, exactly the way you want it -- the difference between Loc and dLoc is not which parent coordinate space it is relative to, but in the other end; i.e. whether the values are absolute (Loc) or relative (deltaLoc) from the parent to the object's OWN coordinate system, i.e. with Loc, the IPO coordinates are inserted as is (from the parent), whereas with dLoc they are added to the object's own offset (still from the parent).
EDIT: Tried to make it less ambigous. :P