I'm proposing to make a full self-consistent set of overrides be the new default <http://code.google.com/p/lilypond/issues/detail?id=3279>
How's that going?
For the record, here's an example of why I want to be able to say "this grob [e.g., BarNumber] is allowed to slide left or right to avoid a collision, but not up or down":
In my opinion, the above is truly — and unnecessarily — unfortunate output.
Cheers,
Kieren.