This may be a very stupid question, for which please forgive me. I've been running BGP for 10+years and I've never had to think about this before.
We run a metropolitan network which uses eBGP with a private AS for each site. (The appropriateness of eBGP is a discussion for another day, thank you.) There may be static routes or an IGP (OSPF) for distributing routes into BGP at the site. We have recently started installing backup links for some of the connections within a site and obviously the IGP just sorts that out.
What would be nice would be to land a back up link from one site into a router in a different AS. Network N, say, normally announced in AS X would now also need to be announced by AS Y. I don't really want to run BGP on the backup link. Is this possible? Would things get confused if N appeared in both ASs? We'd need to pad the announcement from Y to make sure the backup wasn't used in normal traffic (it's lower bandwidth and intended only for emergencies). Should we pad the AS path with Xs or Ys or both? Padding with Xs would make N appear to be in X, but is that sensible or possible?
Or is this just a really stupid idea? :-)