I would like to forward all incoming email for a particular domain to another host via SMTP. Whereabouts would I configure that?

Use this as your first router:

special:
  driver = manualroute
  transport = remote_smtp
  route_list = the.particular.domain the.other.host

You will also need to adjust the ACL for incoming SMTP so that this domain is accepted for relaying. If you are using the default configuration, there is a domain list called relay_domains that is set up for this.