Not tested but I’d look at routing outbound links through a template and using the redirect= variable
Example:
1. Set up a template group called “go” (or whatever you want to name it)
2. Add a tag to the “go” index template to hold the redirect and the URL:
{exp:weblog:entries entry_id="{segment_2}"}
{redirect='{url_custom_field}'}
{/exp:weblog:entries}
... where {url_custom_field} holds the address of the site you’re linking to
3. Update the link href in the directory entry template:
<a href="http://{entry_id_path=go/index}">Link title</a>
... this should output a path with the entry ID in it, eg yoursite.com/go/123
Now when you look at an external link it will display as a URL on your site, but when clicked should just forward the user to the remote site.