I send all external mail to a smart host, but this means that bad addresses also get passed to the smart host. Can I avoid this?

Assuming you have DNS availability, set up a conventional dnslookup router to do the routing, but in the remote_smtp transport set this:

hosts = your.smart.host
hosts_override

This will override the hosts that the router finds so that everything goes to the smart host, but any non-existent domains will be failed by the router.