Why don't you just tag your exportable routes and let them through,
while blocking everything else? You could then build a route-map that
places that tag on all of your inbound BGP customers. This would allow
you to export everything that they send you (allowing them to send the
more specific as Dorian stated) and use a static tag for your aggregate
routes. Then all the more specific routes on your backbone would be
filtered, unless it originated from a BGP customer - who would need to
send the more specific advertisements to each of it's providers and
allow routing to work correctly. I know, that's what we do! If you
don't have the right tag, you don't get off! And I don't have to mess
with any filters after they are set up.
Barry