I have a domain for which some local parts must be delivered locally, but the remainder are to be treated like any other remote addresses.

One possible way of doing this is as follows: Assuming you are using a configuration that is similar to the default one, first exclude your domain from the first router by changing it to look like this:

non_special_remote:
  driver = dnslookup
  domains = ! +local_domains : ! special.domain
  transport = remote_smtp
  ignore_target_hosts = 127.0.0.0/8
  no_more

Then add a second router to handle the local parts that are not to be delivered locally:

special_remote:
  driver = dnslookup
  domains = special.domain
  local_parts = ! lsearch;/list/of/special/localparts
  transport = remote_smtp
  ignore_target_hosts = 127.0.0.0/8
  no_more

The remaining local parts will fall through to the remaining routers, which can delivery them locally.