I don't think filters are a problem for third party routing as long
as the third party routing is not done in secret. If I am sending
you third party routes for someone, and you know it because I tell
you I am and you agree to let me, then you can open your filters to
the source port for those routes. Third party routes that are
being done without the knowledge of the traffic target are a bad
thing and shouldn't be done anyway...
-Dorn