(I will not include Sean's comments here.)
I like what Sean has proposed here a great deal, even with the dificulties
it may introduce. Interestingly, some of the PI/PA multihome arguement goes
away with this scheme. If you want a PA multihome, you pay for the more
specific of the PA block and for teh advertisements of the other blocks. If
you damp the longer prefix, the PA block will still work. If you are stable
but the PA block above you flaps, you are still OK as long as they don't
cause you to flap (which would be fairly likely.)
I can see no disadvantage to this over PI blocks other than renumbering.
Either way you pay to get the undamped advertisement.